Hydrogen

Selina Concise Physics Class 6 ICSE Solutions – Matter

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Selina Concise ICSE Solutions for Class 6 Physics Chapter 1 Matter

Synposis

•  Matter is the substance which occupies space and has mass.
•  The three states of matter are

1.  solid,
2.  liquid
3.  gas.

•  Matter is composed of large number of molecules.
•  A molecule is the smallest particle which can exist freely in nature by itself and it retains the properties of the substance.
•  All molecules of a substance are identical, but the molecules of different substanes are different.
•  A molecule is very small in size (10^-10 m).
•  The molecules are separated from each other with spaces called inter-molecular spacing.
•  The molecules in a substance are held together by the forces acting between the molecules which are called the inter-molecular forces.
•  The force of attraction between the molecules of the same sub¬stance is called the force of cohesion, while the force of attraction between the molecules of two different substances is called the force of adhesion.
•  The forces of cohesion and adhesion are effective only when the separation between the molecules is 10^-9 m. When the separation becomes more, they vanish.
•  The molecules in a substance are not at rest, but they are con¬stantly in motion.
•  In a solid, the molecules are rigid, the inter-molecular spacing is least, the inter-molecular forces are strongest and the molecules remain in their fixed positions. They vibrate to and fro about their mean positions, but they do not leave their positions, so a solid has a definite shape and a definite volume.
•  In a liquid, the molecules are not rigid, the inter-molecular spacing is more than that in solids, the inter-molecular forces are weak and the molecules are free to move within the boundary of the liquid, so the liquid has a definite volume, but it does not have a definite shape.
•  In gases, the molecules are not rigid, the inter-molecular spacing is more than that in solids and liquids, the inter-molecular forces are weakest and the molecules are free to move anywhere in space. So the gas has neither a definite volume nor a definite shape.

Activity – 2

Classify 20 objects around you as solids, liquids and gases

Solids : Ice, Aluminium, Silver, Calcium, Gold, Iron, Sodium, chloride, Sugar, Wood.

Liquids : Water, benzene, chloroform, oil, honey, glycerine, hydrochloric acid, alcohol, dettol.

Gases : Steam, Air, Oxygen, Hydrogen, Chlorine, Nitrogen, Ammonia, Helium, Argon.

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) The molecules of each substance are identical.
Answer. False

(b) The inter-molecular forces are effective at all distances between the two molecules.
Answer. False

(c) The molecules in a substance arc in random motion.
Answer. Tme

(d) In a gas, the molecules can move anywhere in space. .
Answer. Tme

(e) The liquids are less viscous than the gases.
Answer. False

2. Fill in the blanks

(a) All the molecules of a substance are identical.
(b) The inter-molecular spacing is least in solids more in liquids and still more in gases.
(c) The molecular motion in liquid and gas is in zig-zag path.
(d) In a solid, the molecules vibrate to and fro but they remain at their fixed positions.
(e) The inter-molecular forces are the weakest in gases.
(f) A solid exerts pressure downwards on its base.
(g) The gases are least dense.
(h) A solid is most rigid.

3. Select the correct alternative

(a) The diameter of a molecule is approximately

1.  1 cm
2.  10 cm
3.  10^-10 m
4.  1 m

(b) The inter-molecular forces are strongest in

1.  solids
2.  liquids
3.  gases
4.  both (i) and (ii)

(c) The molecules

1.  in solid, liquid and gas, move freely anywhere.
2.  in a solid, move freely within its boundary.
3.  in a liquid, move within its boundary.
4.  in a gas, move only within its boundary.

(d) The solids are

1.  more dense
2.  less dense
3.  least dense
4.  highly compressible

(e) The inter-molecular forces in liquids are

1.  as strong as in solids
2.  stronger than in solids
3.  weaker than in solids
4.  weaker than in gases

4. Match the following columns

B. Short/Long answer questions

Question 1.
Define matter. What is its composition ?
Answer:
Matter is defined as anything which occupies space and has mass. It can be perceived by our sense of smell, touch, sight, hearing and taste.
Matter is composed of tiny particles known as atoms.

Question 2.
Name the three states of matter.
Answer:
The three states of matter are solids, liquids and gases.
Solids —A solid has a definite shape and definite volume.
Example – wood, stone, iron, ice etc.
Liquid —A liquid has a definite volume but not definite shape.
Example — water, juice, milk, oil, etc.
Gases —A gas neither has definite shape nor a definite volume.
Example – air, hydrogen, oxygen, water vapour etc.

Question 3.
What is a molecule ?
Answer:
The smallest unit of matter which can exist independently is called molecule.
Example: Oxygen molecule (O₂) made up of two (O) atoms.

Question 4.
What is the approximate size of a molecule ?
Answer:
Matter is made up of molecules which are very small in size (~10^-9 m).

Question 5.
One litre of water has 6.02 × 10²⁶ molecules. Estimate the size of a molecule.
Answer:
The size of a particle (or molecule of matter is very small. 1 litre of water has 6.02 × 10²⁶ molecules, so the volume of a particle of

Thus the diameter of a water molecule is nearly 1.27 × 10^-9 metre.

Question 6.
What do you mean by inter-molecular spacing ?
Answer:
Intermolecular space — The space between any two consecutive molecules of a substance is called intermolecular space

Question 7.
Describe a simple experiment to illustrate the existence of inter-molecular spacing.
Answer:
Take 100 ml of water in a measuring cylinder. Add 20 gram of salt in water gently and stir it well so as to dissolve the salt well in water. It is noticed that the level of water does not change. It shows that the particles of salt occupy spaces between the particles of water.

Question 8.
What do you mean by inter-molecular forces ?
Answer:
Intermolecular force of attraction — The force of attraction between the molecules (like molecules or unlike molecules) is called intermolecular force of attraction.

Question 9.
What are the forces of cohesion and adhesion ?
Answer:
The force of attraction between the molecules of similar kind is called force of cohesion.
Example: The forces between water molecules.
This force of cohesion keep the molecules of the substance bind together.
The force of attraction between different types of molecules is called force of adhesion.
Example: When a glass filled with water is emptied some water I particles remain stuck to the glass due to the adhesion between water molecules and glass.

Question 10.
State three characteristics of molecules of matter.
Answer:
The particles of matter called molecules, have the following characteristics:

1.  They are very small in size.
2.  They have spaces between them.
3.  They are in constant random motion.
4.  They always attract each other.

Question 11.
State the approximate spacing between two molecules of a matter.
Answer:
The spacing between particles of a matter is called inter-molecular space.

Question 12.
How do the solids, liquids and gases differ in their following properties
(a) Size
(b) Shape
(c) Density
Answer:

Question 13.
The molecules in a substance are in motion. What type of path do they follow ?
Answer:
The particles in a substance are not at rest (in motion),and they move randomly in all possible directions in a zig-zag Path

Question 14.
Describe a simple experiment to illustrate that molecules are not at rest, but they constantly move.
Answer:
Take a beaker. Fill it partly with water. Add some lycopodium powder in the beaker containing water. Stir the contents of the beaker with a glass rod. Take out few drops of this suspension on a glass plate. Place it on the table and illuminate it with a table lamp. Observe the glass plate through a microscope. It is found that the fine particles of lycopodium powder move rapidly in a random manner and their path is zig zag as shown in figure below.

Question 15.
Write down five general properties of solids, liquids and gases.
Answer:
Solids:

1.  The molecules here are very tightly packed having negligible or very less intermolecular space.
2.  They have the strongest intermolecular force of attraction.
3.  The molecules have very small vibration about their mean position i.e. small amplitude.
4.  They have a definite shape and volume.
5.  They are generally hard and rigid.
6.  They are good conductors of heat.

Liquids:

1.  Molecules are less tightly packed.
2.  The intermolecular force of attraction is less than that of solids.
3.  The molecules here can move from one place to another
4.  Do not have any particular shape of their own and thus acquire the shape of the vessel.
5.  A particular quantity of a liquid has a definite volume at a given temperature.

Gases :

1.  The force of attraction between the molecules is the least.
2.  The intermolecular space is the largest.
3.  Neither have a definite shape nor a definite volume.
4.  The molecules move independently.
5.  Worst conductors of heat.

Question 16.
Give the molecular model for a solid and use it to explain why a solid has a definite volume and a definite shape.
Answer:

Here the molecules are very tighty packed that there is no or very less intermolecular space and there is high intermolecular force of attraction (force of cohesion).
The molecules do not move about their mean position and thus solids have a definite shape and volume.

Question 17.
Describe the molecular modcl for a liquid. I-low does it explain that a liquid has no definite shape, but has a definite Volume ?
Answer:

Here the molecules are less tightly packed as compared to solids and also there is lesser force of intermolecular attraction. The intermolecular distance is greater than that in the solids. Thus, they do not have a definite shape but acquire the shape of the vessel in which they are contained but have a definite volume at a given temperature.

Question 18.
A gas has neither a definite volume nor a definite shape. Describe the molecular model to explain it.
Answer:
Here the molecules are far apart from each other i.e. have the greatest intermolecular distance which result into the weakest intermolecular forces of attraction. The molecules as are not

bound by any strong force move about freely and thus gases do not have a definite shape and also do not have any definite volume.

Question 19.
Distinguish between the three states of matter—solid, liquid and gas on the basis of their molecular models.
Answer:
Solids:

Here the molecules are very tighty packed that there is no or very less inteimolecular space and there is high intermolecular force of attraction (force of cohesion).
The molecules do not move about their mean position and thus solids have a definite shape and volume.
Liquids:

Here the molecules are less tightly packed as compared to solids and also there is lesser force of intermolecular attraction. The intermolecular distance is greater than that in the solids. Thus, they donot have a definite shape but acquire the shape of the vessel in which they are contained but have a definite volume at a given temperature.
Gases :

Here the molecules are far apart from each other i.e. have the greatest intermolecular distance which result into the weakest intermolecular forces of attraction. The molecules as are not bound by any strong force move about freely and thus gases do not have a definite shape and also do not have any definite volume.

Question 20.
Distinguish between solids, liquids and gases on the basis of their following properties :
(a) compressibility
(b) fluidity
(c) rigidity
(d) expansion on heating
Answer:

Question 21.
What do you mean by the change of state of matter ? Explain:
(a) the change of a solid into a liquid at a constant temperature, and
(b) the change of a liquid into a gas at a constant temperature.
Answer:
The change in state of matter of a substance from solid to liquid or from liquid to gas is brought by imparting heat energy to it at a constant temperature.
(a) The process of change of a substance from solid state into its liquid state on absorption of heat at a particular temperature, called the melting point, is called melting or fusion i. e.

(b) The process of change of a substance from a liquid state to its gaseous state at a particular temperature, called the boiling point,
is called boiling or vaporisation, i.e.

Selina Concise Physics Class 6 ICSE Solutions – Physical Quantities and Measurement

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Selina Concise ICSE Solutions for Class 6 Physics Chapter 2 Physical Quantities and Measurement

• Synposis
•  The observation of a phenomenon is made possible by using the five senses: sight, smell, touch, hearing and taste.
•  Our senses are not always reliable. They are subjective.
•  Sometimes it is necessary to make an exact measurement.
•  Physics is a science of measurement.
•  We use instruments to get an exact measurement.
•  Four basic measurements in our daily life are: measurement of length, measurement of mass, measurement of time, and measurement of temperature.
•  Measurement is basically a process of comparison of the given quantity with a standard unit.
•  For measuring a quantity we need a unit, and then we find the number of times that unit is contained in that quantity.
•  The unit selected for measurement should be of a convenient size and it must not change ‘with place or time.
•  The distance between two fixed points is called length.
•  The S.I. unit of length is metre (m). Its multiple is kilometre (km), where 1 km = 1000 m. Its sub multiples are centimetre (cm) and millimetre (mm), where 1 cm = 10^-2 m and 1 mm = 10^-3 m.
•  The FPS unit of length is foot (ft) and its sub multiple is inch where 1 ft = 12 inch and 1 ft = 30.48 cm.
•  The most common instruments used to measure length are the metre ruler and the measuring tape which are marked in cm and mm.
•  To measure a length accurately with a metre ruler, the scale should be placed with its markings close to the object and parallel to its r length. The eye is kept in front of and in line with the reading to be taken.
•  The quantity of matter contained in a body is called its mass.
•  The S.I. unit of mass is kilogram (kg). Its multiples are quintal and metric tonne. 1 quintal = 100 kg and 1 metric tonne =10 quintal = 1000 kg. Its sub multiples are gram (g) and milligram (mg) where 1 g = 10 kg and lmg = 10^-6 kg.
•  The FPS unit of mass is pound (lb) where 1 lb = 453.59 g.
•  Mass of a body is measured by using a beam balance or an electronic balance.
•  The interval between two instances or events is called time.
•  The S.l. unit of time is second (s), 1 s 1 / 86400 of a mean solar day. The C.G.S. and F.P.S. unit of time is also second (s).
• The multiple unit of time are minute (mm), hour (h), day and year where 1 min = 60 s, 1 h = 3600 s, 1 day = 86400 s and 1 year = 3.15 × 10⁷ s.
•  The time at any instant is recorded by a pendulum clock or watch and the time interval of an event is measured by using a stop watch or a stop clock.
•  The temperature is the measure of degree of hotness or coldness of a body.
•  The S.I. unit of temperature is kelvin(K), but the common unit of temperature is degree Celsius (°C) and degree fahrenheit (°F).
•  Doctors use a clinical thermometer to measure the patient’s body temperature.
•  The normal temperature of a human body is 37°C or 98.6°E
•  The total surface occupied by an object is called its area. Area is expressed as the product of measured length of two sides.
•  The S.I. unit of area is square metre (m²).
•  One square metre is the area of a square of each side one metre.
•  The bigger (or multiple) units of area are dam², hectare and square kilometre (km²), where 1 dam² =100 m²,1 hectare= 10⁴ m² and 1 km² = 10^6  m2
•  The smaller (or sub multiple) units of area are cm² and mm² where 1cm² = 10^-4 m² and 1 mm² = 10^-6  m²

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) S.I. unit of temperature is Fahrenheit.
Answer. False

(b) Every measurement involves two things – a number and a unit
Answer. True

(c) Mass is the measure of quantity of matter.
Answer. True

(d) The S.I. unit of time is hour.
Answer. False

(e) The area can be expressed as the product of length of two sides.
Answer. Tme

2. Fill in the blanks

(a) The S.I. unit of length is metre of time is second of mass is kilogram.
(b) °C is the unit of temperature.
(c) 1 metric tonne = 1000 kg
(d) The zero mark in Celsius thermometer is the melting point of ice
(e) The thermometer used to measure the human body temperature is called the clinical thermometer.
(f) The normal temperature of human body is 37 °C or 98.6 °F.
(g) The mass of an object is measured with the help of a beam balance.

3. Match the following columns

4. Select the correct alternative

(a) The symbol of degree celsius is

1. °C
2.  °F
3.  K
4.  °K

(b) lO mm is equal to

1.  1cm
2.  1m
3.  10dm
4.  10cm

(c) The amount of surface occupied by an object is called its:

1.  volume
2.  area
3.  mass
4.  length

(d) A metre ruler is graduated in:

1.  m
2.  cm
3.  mm
4.  km

(e) A thermometre is graduated in:

1.  kelvin
2.  °C
3.  g
4.  cm

B. Short/Long Answer Questions

Question 1.
What is measurement ? How is a measurement expressed ?
Answer:
Measurement is a comparison of an unknown quantity with a known fixed quantity of the same kind.
The value obtained on measuring a quantity is called its magnitude. The magnitude of a quantity is expressed as numbers in its unit.

Question 2.
State two characteristics of a unit.
Answer:
Two characteristics of a unit are

1.  It should be of convenient size.
2.  It must be universally accepted, i. e. its value must remain same at all places and at all times.

Question 3.
Name four basic measurements in our daily life.
Answer:
In our daily life we measure the following four basic physical quantities.

1.  Length
2.  Mass
3.  lime
4.  Temperature

Question 4.
What are the S.I. units of

1.  mass
2.  length
3.  time and
4.  temperature. Write their names and symbols.

Answer:
S.I. units are as follows

Question 5.
Define one metre, the S.I. unit of length. State its one multiple and one sub multiple.
Answer:
One metre is defined as the distance travelled by light in air in \(\frac { 1 }{ 299,792,458 }\) of a second
Multiple of metre = Kilometre
Submultiple of metre = Centimetre

Question 6.
Convert the following quantities as indicated
(a) 12 inch = ft
(b) 1 ft = cm
(c) 20 cm = m
(d) 4.2 m = cm
(e) 0.2 km = m
(f) 0.2 cm = mm
(g) 1 yard = m
Answer:
(a) 12 inch 1 ft
(b) 1 ft = 30.48cm
(c) 100 cm = 1m

Question 7.
(a) Describe in steps how would you measure the length of a pencil using a metre rule. Draw a diagram if necessary.
Answer:
To measure the length of a pencil using a metre rule, place metre rule with its marking close to the object. Let PQ be a pencil.
The end P of the pencil coincides with the zero mark on the ruler. The end Q of the pencil is read by keeping the eye at the position ‘B’ vertically above the end Q. So the length of pencil is 4.3 cm.

(b) Explain with an example how you will use the metre ruler in part (a) if the ends of ruler are broken.
Ans. The ends of the ruler get damaged with use and its zero mark may not be visible. To measure the length of an object with such a ruler, the object is placed close to a specific markings on the ruler and positions of both ends of the object are read on the ruler.

The difference of the two readings gives the length of the object. In fig. the reading on ruler at the end X is 1.0 cm and at the end Y is 4.3 cm. So the length of the rod XY is 4.3 — 1.0 = 3.3 cm.

Question 8.
Name the device which you will use to measure the perimeter of your play ground. Describe in steps how you will use it.
Answer:
We will use a measuring tape to measure the perimeter of our playground.
To measure the length of playground the tape is spread along the length of the curved area.

Question 9.
The diagram below shows a stick placed along a metre RULER. The length of the stick is measured keeping the eye at positions A, B and C.

Answer:
(a) Write the length if stick PQ as observed, for each position of the eye. Are they all same?
Ans. Length of stick PQ from
Position A =3.4 cm
Position B = 3.2 cm
Position C = 3.00 cm
No they are not same.
(b) Which is the correct position of the eye? Write the correct length of the stick.
Ans. ‘B’ is the correct position of the eye. Correct length of the stick PQ = 3.2 cm

Question 10.
Define mass. State its (1) S.I. (2) C.GS and (3) EP.S. units. How are they related ?
Answer:
The mass of a body is the quantity of matter contained in it. The S.I. unit of mass is kilogram. In short form, it is written as kg.
In C.GS. system, the unit of mass is gram, (symbol g).
In F.P.S. system, the unit of mass is pound (symbol lb)

Question 11.
Convert the following quantities as indicated:
(a) 2500 kg = ………. metric tonne.
(b) 150 kg = quintal
(e) 10 lb = ………. kg
(d) 250 g = … .kg
(e) 0.01 kg = ………. g
(f) 5 mg = ………. kg
Answer:

Question 12.
Name the instrument which is commonly used to measure the mass of a body. State how is it used ?
Answer:
Instrument commonly used to measure the mass of a body, is the beam balance.

When we hold up the balance, we observe that when there is nothing on either pan, the beam is horizontal. The body whose mass is to be measured is placed on the left pan. The standard weight are put on the right pan. They are so adjusted that the beam is again horizontal on holding the balance up. The total of the standard weights gives the mass of the given body.

Question 13.
Define one kilogram, the S.I. unit of mass. How is it related to (i) quintal (ii) metric tonne and (iii) gram.
Answer:
The mass of 1 litre of water at 4 °C is taken as 1 kilogram
1 quintal = 100 kg
1 metric ton = 10 quintal = 1000 kg

Question 14.
Name and define the S.I. unit of time. How is it related to (i) minute (ii) hour, (iii) day and (iv) year ?
Answer:
The S.I. unit of time is second. In short form we write it as ‘ S ’.
One second is the time interval between the two consecutive ticks that you hear from pendulum wall clock.
1 min = 60 s
1 h = 60 min. = 3600 s.
1 day = 24 h = 86400 s.
1 year = 365 days = 3.15 × 10⁷ s.

Question 15.
Name two devices used to measure the short time interval of an event.
Answer:
Two devices used to measure the time interval of an event are

1.  StopWatch
2.  Stop Clock

Question 16.
Express in second

1.  3 minute 15 second and
2.  5 hour 2 minute 5 second.

Answer:

1.  3 minute = 15 second
   1 minute = 60 second
   3 minutes 15 second = 60 × 3 + 15
   = 180 + 15
   = 195 seconds
2.  1 minute =60 second
   2 minutes =2 × 600 = 120 second …(1)
   1 hour 3600 second
   5 hour 3600 × 5 = 18000 second …(2)
   5 hour 2 minutes and 5 second
   = 18000 + 120 + 5 = 18125 seconds

Question 17.
What does the temperature measure ?
Answer:
Temperature measures the degree of coldness and hotness of a body.

Question 18.
Name the

1.  S.I. unit and
2.  one common unit of temperature. Write their symbols also.

Answer:
The S.I. unit of temperature is kelvin (symbol K).
Common unit of temperature is degree centigrade (symbol °C)

Question 19.
Name the instrument used for measuring of the temperature of a person. Draw its labelled neat diagram.
Answer:
The temperature is measured with a thermometer.

Question 20.
Write the temperature of (i) melting ice (ii) boiling water.
Answer:
The temperature of

1.  melting ice = 0 °C
2.  boiling water = 100 °C

Question 21.
What is a clinical thermometer ? State its special feature. Draw a labelled neat diagram of a clinical thermometer showing the range of temperature marked on it.
Answer:
Doctors use a special thermometer called the clinical thermometer for measuring the temperature of the patient’s body. This thermometer has the markings from 35°C to 42°C. It has a slight bend or kink in the stem just above the bulb. This kink is called the constriction. This constriction prevents the mercury from falling back all by itself. The temperature of a healthy person is 37°C. This temperature is marked by a red arrow.

Question 22.
What is the normal temperature of the human body ? How is it indicated in a clinical thermometer ?
Answer:
Normal temperature of a human body is 37°C or 98.6 °F.
To measure the temperature of a patient’s body, its bulb is kept either below the tongue or under the arm’s pit of the patient for about a minute. Then the thermometer is taken out and its reading is noted. When the temperature of patient’s body is above 37°C, he is said to suffer with fever.

Question 23.
Can a clinical thermometer be used to measure the temperature of the boiling water ? Give reason for your answer.
Answer:
No, a clinical thermometer cannot be used to measure the temperature of boiling water. ,
The reasons are

1.  It has a very small range.
2.  It can break on cooling and on excess heating.

Question 24.
Explain the term ‘area of a surface’.
Answer:
The total surface occupied by an object is called its area or surface area.

Question 25.
Name the S.I. unit of area and define it.
Answer:
The S.I. unit of area is square metre or meter² which in short form is written as m².

Question 26.
How are the units

1.  square yard
2.  hectare
3. km²
4. cm²
5. mm² related to the S.I. unit of area ?

Answer:

Question 27.
Explain how you will measure the area of (i) a square (b) a leaf?
Answer:
The area of a square can be calculated by using the following formula –
1. Area of square of side l
= side x side
= l × l = L² .
The area of a leaf is obtained by using a graph paper. A graph paper has small squares of each side 1 mm. The area of each big square is 1 cm².
Procedure: Place the leaf on graph paper. Draw its outline on the paper and remove it. Now count the number of complete squares. To this add the number of incomplete squares which are half or more than half. Ignore the squares which are less than half. Thus,
Approximate area = (No. of complete squares + no. of half or more than half of incomplete squares) × area of one square.

Selina Concise Physics Class 7 ICSE Solutions – Electricity and Magnetism

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Physics. You can download the Selina Concise Physics ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Physics for Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 7 Physics Chapter 7 Electricity and Magnetism

• Points to Remember
•  Some materials behave in a particular manner showing magnetic properties.
•  A freely suspended bar magnet always point in North-South direction.
•  Like poles repel each other while unlike poles attract each other.
•  Magnet is neutral at its centre and has maximum magnetic effects at its ends called the poles.
•  Poles always exist in pairs with opposite polarities and can never be isolated.
•  Examples of magnetic substances are:- iron, cobalt, nickel.
•  Examples of non-magnetic substances are:- wood, plastic, aluminium, copper.
•  The poles are not exactly the ends of the magnet but thev are slightly inside.
•  For a given magnet, both the poles are of equal strength.
•  Earth’s magnet has its south pole situated near the geographic north.
•  Electromagnets are the temporary magnets which are made up of soft iron core with a winding of insulated copper wire.
•  Freely suspended current carrying solenoid always rests in north- south direction.
•  A direct current source has the fixed positive and negative terminals, e.g. a battery.
•  Hans Christian Oersted discovered that if an electric current is passed through a conductor, a magnetic field is developed around it.
•  The study of magnetic effects produced due to electric current is known as electromagnetism.
•  The direction of magnetic field due to a straight current carrying conductor is obtained by any of the following rules:
  (a) Right Hand Thumb Rule.
  (b) Right Hand Cork Screw Rule.
•  The property due to which a changing magnetic field within a closed conducting coil induces electric current in the coil is called electromagnetic induction.
•  The current produced in a closed coil when magnetic lines of force rapidly change within it is called the induced current.
•  The symbol for alternating current is 0.
•  The strength of induced current can be increased by increasing the
  (a) the number of turns in the coil
  (b) strength of the magnet used
  (c) relative speed between the magnet and closed coil.
•  We cannot think of modem life without electricity. We light our homes and other places of work with electricity.
•  It is used to run electric fans, televisions, geyser, electric irons, room heaters, refrigerators, music system etc.
•  Cell is a primary source of electricity. A combination of two or more cells is a battery.
•  Some other sources of electricity are generator and solar cells.
•  The path along which an electric current flows is called a circuit.
•  Electricity has the following effects
  (a) Heating effect
  (b) Magnetic effect
  (c) Chemical effect
  (d) Mechanical effect
•  When an electric path is complete is called closed circuit and the path with a break is called open circuit.
•  The substances which allow the electricity to flow through them are called conductors, e.g. metals, human body etc.
•  The substances which do not allow electricity to flow through them are called insulators, e.g. wood, paper, glass etc.
•  The consumption of electricity is calculated from the meter in kWh.
•  Electric fuse is a device which limits the current in an electric circuit.
•  All electrical appliances are connected in parallel in household circuits.
•  We should be cautious in using electricity. Certain precautions should be taken before working on an electrical gadget or circuit.

Activity 6

List five such electrical gadgets in your house in which electromagnet is used.

1. ……………………
2. ……………………
3. ……………………
4. ……………………
5. ……………………

Answer.

1. Computer
2. Electric motor
3. Fan, Toaster
4. Refrigerator
5. Television
6. Electric Bell

Activity 9

Answer.

Test Yourself

A. Objective Questions 

1. Write true or false for each statement

(a) A current carrying coil when suspended freely can rest in any direction.
Answer. False.

(b) A coil carrying current behaves like a magnet.
Answer. True.

(c) In an electromagnet, the core is made up of copper.
Answer. False.

(d) An electric bell uses an electromagnet.
Answer. True.

(e) An electromagnet with soft iron core is a temporary magnet.
Answer. True.

(f) We use cell as the source of electricity to run an electric immersion rod.
Answer. False.

(g) A torch bulb glows if the terminals of the bulb are connected to the terminals of a cell by the metallic wire.
Answer. True.

(h) Wool is a conductor of electricity.
Answer. False.
Wool is a insulator of electricity.

(i) Silver is an insulator of electricity.
Answer. False.
Silver is good conductor of electricity.

(j) Our body is a conductor of electricity.
Answer. True.

(k) For a circuit to be complete, every part of it must be made up of conductors.
Answer. True.

(l) All metals are conductors of electricity.
Answer. True.

(m) The switch should not be touched with wet hands.
Answer. True.

(n) A switch is an on-off device in an electric circuit.
Answer. True.

2. Fill in the blanks

(a) A magnet has two poles.
(b) Like poles repel each other and unlike poles attract.
(c) An electromagnet is used to separate large mass of iron scrap.
(d) The strength of magnetic field of an electromagnet is increased by inserting a core of soft iron.
(e) In a torch we use dry cell as the source of electricity.
(f) To light a table lamp and to run a refrigerator, we use mains as the source of electricity.
(g) A group of two or more cells is called a battery.
(h) Conductors pass electricity through them.
(i) Insulators do not pass electricity through them.

3. Match the following

4. Select the correct alternative

(a) A freely suspended magnet rests in

1.  east-west direction
2.  north-south direction
3.  north-east direction
4.  north-west direction.

(b) Electromagnets are made up of

1.  steel
2.  copper
3.  brass
4.  soft iron.

(c) An electromagnet is used in

1.  electric oven
2.  ammeter
3.  electric bell
4.  radio set.

(d) The purpose of armature in an electric bell is

1.  to make and break the circuit
2.  to produce sound
3.  to produce magnetic field
4.  to provide spring action.

(e) In a torch, the source of electricity is

1.  the bulb
2.  the switch
3.  the cell
4.  the mains.

(f) Electricity can flow through

1. wood
2.  rubber
3.  plastic
4.  copper wire.

(g) Electricity does not flow through

1.  human body
2.  animals body
3.  rubber
4.  silver.

(h) We should not touch the switch with wet hands otherwise

1.  electricity may pass through our body
2.  electricity may not pass through the appliance
3.  circuit may break
4.  the switch may get off.

B. Short/Long Answer Questions

Question 1.
State two properties of a bar magnet.
Answer:
Properties of a bar magnet

1.  Attractive property: A magnet attracts small pieces of iron, cobalt or nickel.
2.  Directive property: A magnet when suspended freely, always point towards North and South direction.
3.  Like poles, i.e. North and North or South and South poles repel each other.
4.  Unlike poles i.e. North and South attract each other.
5.  Poles always exist in pairs, i.e. poles of a magnet cannot be separated.

Question 2.
How will you test whether a given rod is a magnet or not?
Answer:
Aim— To test whether a given rod is a magnet or not.
Apparatus —

1.  rod to be tested
2.  a bar magnet
3.  a stand
4.  a thread

Procedure —

1.  Suspend the rod to be tested with a thread on the stand.
2.  Bring the bar magnet near the rod with its north pole towards the rod’s end.
3.  Observe.

Inference — In every case, the magnet comes to rest in a north- south direction which shows its directive property.
Observation and Inference —
— If the rod is attracted towards the bar magnet, the rod may be magnet or not.
— But if the rod remains in the same direction of rest as in the start of the experiment, it show it is not a magnet.

Question 3.
How will you test whether a given rod is made of iron or not?
Answer:
Bring a magnet near the rod if the rod is attracted by the magnet then it will be made of iron else not.

Question 4.
You are given two similar bars. One is a magnet and the other is of soft iron. How will you distinguish and identify them ?
Answer:
Take first bar and suspend it in a stand with the help of a thread. So that it is free to rotate in horizontal plane. Note the direction in which it sets itself. If the direction is North and South it may be magnet. Again rotate it, if this time again it sets itself in north and south direction, it is a magnet, otherwise, it is iron.
Now repeat above experiment with second bar and in the same way find it if it sets always in north and south direction then it will be a magnet.

Question 5.
You are given a magnet. How will you use it to find north-south direction at a place?
Answer:
The earth is a huge magnet in itself with its North and South poles. The North and South of the place can easily be detected with the help of a bar magnet. A freely suspended bar magnet always rest in the geographic N-S direction. Since the South pole of the earth’s magnet is closer to the geographic North, the North pole of the suspended magnet will always rest in the geographic North, and the South pole of the freely suspended magnet will point towards geographic South.

Question 6.
Describe a simple experiment to illustrate that like poles of two magnets repel each other while the unlike poles attract.
Answer:
Like poles repel and unlike poles attract.
Two like poles (both North poles or both South poles) repel each other. Two unlike poles (one North pole and the other South pole) attract each other. This can be demonstrated by the following simple experiment.
Take two bar magnets A and B. Suspend one magnet A with a silk thread from a support so that it is free to swing. The magnet A will come to rest in the North-South direction. The North pole of the magnet A is in the North direction and its South pole is in the South direction. Now holding the other magnet B in your hand if you bring its North pole near the North pole of the suspended magnet A as shown in figure you will observe that the two poles repel each other. Care is taken that the two magnets do not touch each other.

Now if you bring the South pole of the magnet B near the north pole of the suspended magnet A as shown in figure without touching it, you will observe that the two poles attract each other.

The above experiment shows that the like poles repels each other while the unlike poles attract each other.

Question 7.
“Poles exist in pair”. Comment on this statement.
Answer:
The magnetic poles always exist in pairs. It is not possible to separate the two poles of a magnet.
If a bar magnet is broken at the middle in two parts each part is found to be a magnet. Each part has the property to attract the small iron pieces. Each part rests in the North-South direction when suspended such as to swing freely. This shows that the new poles are formed at the broken ends.
If these pieces are broken again and again, each part will still found to be a complete magnet. Each part contains both the poles (N-pole and S-pole). Thus, the two poles of a magnet exist simultaneously.

Question 8.
What is a magnetic compass ? State its use.
Answer:
Magnetic compass is a device which is used to locate the direction of a place. It always rests in a North-South direction. It is used in the navigators in ships, submarines, aeroplanes etc.

Question 9.
Explain the meaning of the term magnetic field.
Answer:
The space around the magnet where its influence can be experienced is known as magnetic field. This field is formed by the magnetic lines of force which run from the North pole to the South pole. These lines can be found to be maximum crowded at the two ends of the magnet which are the poles i.e. the North pole and the South pole.

Question 10.
What is an electromagnet ?
Answer:
An electromagnet — An electromagnet is a temporary magnet which behaves as a magnet when electric current is passed through the insulated copper wire and loses its magnetism when current is stopped. It has a soft iron piece called the core with an insulated copper wire wound on it.

Question 11.
Name the material of an electromagnet.
Answer:
Iron bar, insulated copper wire, battery.

Question 12.
Draw a labelled diagram to make a soft iron bar as an electromagnet. Describe in steps the procedure.
Answer:
Usually, the electromagnets are made in two shapes :
(1) bar or I shaped magnet and (2) horse shoe or U shaped magnet.

1.  To make a fiar or I shaped electromagnet: Take a soft iron bar PQ and wind a thin insulated copper wire around the bar. Connect a cell or a battery B, and a key K in series between the ends of the coil. The circuit diagram is shown in figure.
   
   When key K is closed, current passes through the winding of the coil and the bar becomes a magnet. As the key K is opened, the current stops flowing in the coil and the bar loses its magnetism. Thus, the bar behaves like an electromagnet.
2.  To make a horse shoe or U shaped electromagnet: Take a U shaped soft iron piece. Wind a thin insulated copper wire on its arms such that the winding in the two arms is in opposite direction. In figure winding in the arm A starts from the front and is in clockwise direction (when seen from the bottom).
   On reaching the upper end of the arm A, winding starts from the back at the top of the arm B and is in anticlockwise direction. Connect a battery B and a key K between the two ends of the wire.
   

Question 13.
You are given a U shaped soft iron piece, insulated copper wire and a battery. Draw a circuit diagram to make a horse shoe electromagnet.
Answer:
End A becomes S-pole and B becomes N-pole.

Question 14.
Name two factors on which the strength of magnetic field of an electromagnet depends.
Answer:
The magnetic field of an electromagnet (I or U-shaped) can be increased by the following two ways :

1.  By increasing the number of turns of winding in the solenoid.
2.  By increasing the current through the solenoid.

Question 15.
State two ways by which the strength of magnetic field of an electromagnet can be increased.
Answer:
The magnetic field of the electromagnet can be increase in the following two ways :

1.  By inserting a rod of soft iron or steel inside the cylindrical tube. This rod is called the core.
2.  By increasing the total number of turns of the coil.

Question 16.
State two common uses of electromagnets.
Answer:
Uses of electromagnet —

1.  In electrical appliances such as electric bell, fan etc.
2.  In lifting heavy loads of iron scrap.
3.  To remove tiny particles of iron from the wound.
4.  In loading furnaces with iron.
5.  In separation of magnetic substances from the non-magnetic substances.

Question 17.
Name a domestic device in which an electromagnet is used.
Answer:
Electromagnet is used in ELECTRICAL APPLIANCES like ELECTRIC BELL, RADIO, T.V., FAN and MOTORS etc.

Question 18.
Draw a neat and labelled diagram of an electric bell and describe its working.
Answer:
Working of the bell— Principle of working of electromagnetism When the switch is pushed on, the circuit gets completed and current stalls flowing through the U-shaped electromagnet which creates magnetic field in the core. This attracts the iron armature. Now when the armature moves towards the electro-magnet, the hammer strikes the gong and the bell rings. But as the armature
moves towards the electromagnet, the contact with the adjustment screw breaks which breaks the closed circuit and stops the current. Now when there is no current there is no electromagnetism and the armature returns to the original position. This making and breaking of the circuit of the electromagnet continues as long as the button remains pressed.

Question 19.
The incomplete diagram of an electric bell is given in fig. Complete the diagram and label its different parts.

Answer:

Question 20.
What is declination ? Draw a diagram to show the angle between the declination and true direction of geographic north.
Answer:

Magnetic declination is the angle of the horizontal plane between the magnetic North and the geographic North (or true North). This angle is shown in figure by symbol θ.
The angle of declination is different at different places on the earth surface and it also changes at a place with time. The declination is taken positive if the magnetic North is towards the east of the true North as in figure and is negative if the magnetic north if towards the west of the true North.

Question 21.
Define the term current.
Answer:
An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire.
The S.I. unit of electric current is the ampere.

Question 22.
Name four appliances which work using electricity.
Answer:

1.  an electric iron
2.  an electric heater
3.  an electric kettle
4.  an immersion rod

Question 23.
Name two sources of electricity.
Answer:

1.  dry cell and battery
2.  generator and solar cell

Question 24.
What is a battery?
Answer:
If we use a group of two or more cells, it is called a battery. A battery is used where we require more electricity.

Question 25.
What is an electric circuit?
Answer:
For a smooth flow of electric current, a complete circuit is needed. This is known as electric circuit.

Question 26.
Describe an experiment to show that electricity flows only if the circuit is complete and it does not flow if the circuit is incomplete.
Answer:
Take two torch bulbs A and B. Connect them to a cell through a switch as shown in fig. The bulbs are said to be in series. Close the switch (i.e., the circuit it completed), you will see that both the bulbs glow.

Now take out the connection of the bulb B as shown in fig. Now close the switch, you will observe that the bulb A does not glow because the circuit is now incomplete.

Now replace the bulb B by a fused bulb fig. and close the switch. Again you will see that the bulb A does not glow. This is because the circuit being in series, is still incomplete.

Question 27.
You are provided with a torch bulb, a cell and two plastic coated f metal wires. Draw a diagram to show a complete circuit to light the bulb.
Answer:
Take two bulbs A and B. Connect them through switches S₁ and S₂ in parallel as shown in fig. Close both the switches. You will see that both the bulbs glow.

Question 28.
In which of the following case the bulb will glow :

1.  Only one terminal of a cell is joined with a metal wire to one terminal of the bulb.
2.  Both terminals of the bulb are joined with two metal wires to one terminal of the cell.
3.  One terminal of the cell is joined to one terminal of the bulb and other terminal of the cell to the other terminal of the bulb.
   Answer:
   The bulb will glow in (3) case i.e.

Question 29.
Distinguish between conductors and insulators of electricity. Give two examples of each.
Answer:
Conductors

1.  Conductors are those substances which allow electricity to flow through them.
2.  e.g. all metals, human body.
   Insulators
   Insulators are those which do not allow electricity to pass through them.
   e.g. wood, paper, glass.

Question 30.
Select conductors and insulators from the following :
Glass, silver, copper, wood, paper, pure water, impure water, aluminium, iron, leather, plastic, steel, human body and ebonite.
Answer:
Conductors — Silver, copper, impure water, aluminium, iron, steel, human body.
Insulators — Glass, wood, paper, pure water, leather, plastic and ebonite.

Question 31.
The following diagram shows four circuits A, B, C and D. Each circuit has a cell and a torch bulb. Name the circuits in which the bulb will glow ? Give a reason to your answer

Answer:
The bulb will glow in circuit (D).
This is because copper is the best conductor of electricity as compared to aluminium. Silk is a non-conductor of electricity.

Question 32.
The diagram given below shows a bulb connected with a cell having terminals A and B. Mark the direction of current in the bulb.

Answer:

Current always flows from +ve terminal to the -ve terminal of a cell.

Question 33.
State the function of each of the following in an electric circuit and draw its symbolic representation: (1) Switch and (2) Cell.
Answer:

1.  Switch – A switch or key is used to put the circuit on and off. fig. shows the symbol of a switch or key when it is open (to put the circuit off and when it is closed (to put the circuit on)
   
2.  Cell – A cell or a group of cells is generally used as a source of electricity. A positive (+) and a negative (-). It is represented by the two vertical lines of unequal lengths. The long vertical line represents the positive terminal and the short line represents the negative terminal as shown in fig.
   

Question 34.
Draw a circuit diagram for a bulb connected to a cell with a switch. Mark arrow in the diagram to indicate the direction of flow of current.
Answer:

Question 35.
In which arrangement are the appliances connected in the electric circuit of our homes, Series or Parallel ? Give one reason for your answer.
Answer:
Parallel circuit: When the circuit is in parallel, the appliances work independently. This is the reason that in our household wiring system, all the circuits are in parallel. Every appliance when put on, works on its own without the interruption of the other appliance.

Question 36.
State two precautions that you must take when switching on an electric circuit.
Answer:
Precautions to be taken before the circuit is switched on
Before the circuit is switched on, following precautions must be taken :

1.  See that all the components of the circuit are properly connected.
2.  See that the connection wire is tightly connected to each appliance or component.
3.  Do not touch the switch or any component with wet hands.

Selina Concise Physics Class 7 ICSE Solutions – Light Energy

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Selina Concise ICSE Solutions for Class 7 Physics Chapter 4 Light Energy

• Points to Remember
•  Light is a form of energy which helps us to see objects.
•  Light always travels in a straight line in the form of rays.
•  Light sources are either natural or artificial.
•  The sun and stars are natural sources of light.
•  A bulb, a candle etc. are artificial sources of light.
•  The bodies which emit light themsleves are called Luminous Bodies, e.g. sun, star.
•  The bodies which do not emit light are called non-luminous bodies, e.g. wood, brick etc.
•  Objects are of three types, transparent, translucent or opaque.
•  The pinhole camera is a simple application of the rectilinear propagation of light.
•  When an object blocks light, it casts a shadow.
•  Eclipses are formed due to formation of shadows.
•  Solar and lunar eclipses are the examples of formation of shadow in nature.
•  An eclipse is the partial or complete hiding of one heavenly body by shadow of another.
•  When light strikes a polished surface it comes back in the same medium, is called reflection of light.
•  A straight highly polished, smooth and reflecting surface is known as a plane mirror.
•  Mirrors are of two types (a) plane mirrors (b) spherical mirrors.
•  Smooth and polished surface like a mirror causes reflection and is called a regular reflection.
•  Rough or diffused surface causes an irregular reflection.
•  According to first law of reflection. The incident ray, the normal and the reflected ray all lie in the same plane.
•  According to the second law of reflection, the angle of incidence is always equal to the angle of reflection.
•  The phenomenon due to which the left side of an object appears to be the right side of the object and right side appears left. This is known as lateral inversion.
•  Image is of two types (a) Real image (b) Virtual image.

Test Yourself

A. Objective Questions 

1. Write true or false for each statement

(a) The image formed by a plane mirror is real.
Answer. False.
The image formed by a plane mirror is virtual.

(b) When a light ray is reflected from a wall, the angle of incidence is not equal to the angle of reflection.
Answer. False.
When a light ray is reflected from a wall, the angle of incidence is equal to the angle of reflection.

(c) The image of the right hand in a plane mirror looks like that of a left hand.
Answer. True.

(d) The image formed by a plane mirror is upright.
Answer. True.

(c) The image formed by a plane mirror can be obtained on a screen.
Answer. False.
The image formed by a plane mirror cannot be obtained on a screen.

(f) The objects are seen around us due to irregular reflection of light.
Answer. True.

(g) The speed of light in vacuum is 3 × 10⁸ ms^-1.
Answer. True.

(h) A rose appears red in light of all the colours.
Answer. False. A rose appears red in white light.

(i) A black paper absorbs light of all the colours and reflects none.
Answer. True.

(j) The primary colours are red, blue and green.
Answer. True.

2. Fill in the blanks

(a) Angle of incidence = angle of reflection
(b) The incident ray, the reflected ray and the normal lie in one plane
(c) The image formed by a plane mirror is at a distance behind the mirror as the object is in front of it.
(d) The image formed by a plane mirror is erect and virtual.
(e) We are able to see the objects around us due to irregular reflection.
(f) A virtual image cannot be obtained on a screen.
(g) One surface of mirror is made opaque by silvering it followed by a thin coating of paint of lead oxide. .
(h) A plane mirror does not reflect 100 percent light falling on it.
(i) The colour of an opaque object is the colour of light which it reflects.
(j) Magenta, cyan and yellow are the secondary colours.

3. Match the following

4. Select the correct alternative

(a) A man standing in front of a plane mirror finds his image to be at a distance of 6 metre from himself. The distance of man from the mirror is

1. 6 m
2. 3 m
3. 2 m
4. 12 m

(b) The angle between the incident ray and the ray reflected from the plane mirror is 70°. The angle of incidence will be :

1. 70°
2. 30°
3. 35°
4. 90°

(c) The image formed by a plane mirror is

1. virtual and inverted
2. virtual and of same size
3. real and inverted
4. real and of same size

(d) The angle of incidence on a plane mirror is 30°.The angle of reflection will be:

1. 30°
2. 60°
3. 15°
4. 0°

(e) The angle of incidence on a plane mirror is 30°. The angle between the incident ray and the reflected ray is

1. 30°
2. 15°
3. 60°
4. 90°

(f) The property due to which a light ray striking a surface is returned back into the same medium is called

1. refraction
2. reflex action
3. reflection
4. regression

(g) A ray of light after reflection from a mirror is known as

1. reflected ray
2. normal
3. incident ray
4. refracted ray

(h) The speed of light is maximum in

1. glass
2. water
3. air
4. wood

(i) A red rose is seen in green light. It will appear.

1. red
2. blue
3. yellow
4. black

(j) The primary colours are

1. Red, Blue and Yellow
2. Magenta, Yellow and Cyan
3. Red, Blue and Cyan
4. Blue, Green and Red

B. Short/Long Answer Questions

Question 1.
What do you mean by the term reflection of light ?
Answer:
Reflection of light— When light strikes a polished surface it comes back in the same medium, is called reflection of light.

Question 2.
How is a plane mirror made ?
Answer:
To make a plane mirror, a thin piece of glass is taken. One surface of the mirror is made opaque by silvering it. On the top of that, another thin coating of red lead oxide is given which protects the silvering of the mirror.

Question 3.
Explain the following terms:
Incident ray, Reflected ray, Angle of incidence, Angle of reflection, Normal.
Answer:
Incident ray— The ray of light falling on the surface AB is called the incident ray. In figure PN is the incident ray.
Reflected ray— The incident ray bouncing back in the same medium after striking the reflecting ourface is called reflected ray. In figure NQ is the reflected ray.

Angle of incidence— The angle formed between the incident ray and the normal is the angle of incidence. (PNM is the angle of incidence.)
Angle of reflection— The angle formed between the normal and the reflected ray is called angle of reflection (MNQ is the angle of reflection)
Normal— It is the line drawn perpendicular to the reflecting surface at the point of incidence. MN is the normal.

Question 4.
Draw a diagram showing the reflection of a light ray from a plane mirror. Label on it the incident ray, the reflected ray, the normal, the angle of incidence i and the angle of reflection r.
Answer:
AO is the incident ray
OB is the reflected ray

ON is the normal
∠AON is the angle of incidence
∠NOB is the angle of reflection.

Question 5.
State the two laws of reflection of light.
Answer:
Laws of reflection—
(i) The incident ray, normal and the reflected ray all lie in the same plane.
(ii) The angle of incidence is equal to the angle of reflection.

Question 6.
Describe an experiment to verify the laws of reflection of light. Ans. Laws of reflection.
Answer:
(i) The incident ray, the reflected ray and the normal at the point of incidence, lie in the same plane.
(ii) The angle of incidence and angle of reflection are equal i.e. ∠i = ∠r.
Verification
Take a wooden drawing board and fix a white sheet of paper on it. In the middle of paper draw a straight line KK\ Mark a point B on it. Draw a perpendicular BN. Place a mirror XX’ on line KK’ such that polished side of mirror is along the line. Hold-the mirror in the mirror holder.

Fix two steel pins P and Q on the straight line AB atleast 10 cm apart. Look for the images of the pins P and Q and fix two pins P Q’ such that P’, Q’ and images of P and Q are all in the same straight line. Remove the pins and draw small circles around the pin pricks.
Remove the mirror also. Join P’Q’ and produce the straight line to meet at B.
Measure ∠ABN = i and ∠CBN = r.
It is found that ∠i = ∠r. This proves that Angle of Incidence is equal to Angle of Reflection.
As the incident ray, the reflected ray and the normal lie in the plane of paper, therefore, they lie in the same plane.

Question 7.
A ray of light falls normally on a plane mirror. What is the angle of incidence ?
Answer:
Angle of incidence is 0°. Since angle of incidence is’the angle between incident ray and normal. Direction of reflected ray is along BA opposite to the direction of incident ray.

Question 8.
Draw a diagram to show the reflection of a light ray incident normally on a plane mirror.
Answer:
AO is the incident ray
OB is the reflected ray

ON is the normal
∠AON is the angle of incidence
∠NOB is the angle of reflection.

Question 9.
The diagram in Fig. shows an incident ray AO and the reflected ray OB from a plane mirror. The angle AOB is 30°. Draw normal on the plane mirror at the point O and find :
Answer:
(i) the angle of incidence
(ii) the angle of reflection

ON is normal on the plane mirror at point O
ON is perpendicular on a plane mirror
Angle of incidence ∠i = ∠AON
and angle of reflection ∠r = ∠BON
Since, ∠i – ∠r
∠AOB = 30°
⇒ ∠AON + ∠BON = 30°
⇒∠i + ∠i – 30°
⇒ 2 ∠i =30°
⇒ ∠i = 30 / 2 = 15°
∴Angle of incidence = ∠i = 15°
and Angle of reflection ∠i = 15°

Question 10.
In the following diagrams, measure and write the angle of incidence and draw the reflected ray in each case.
Answer:

Question 11.
The diagram in fig. shows an incident ray AO and the normal ON on a plane mirror. Draw the reflected ray. State the law you use to draw the direction of the reflected ray.

Answer:

Law of reflection of light is used to draw the direction of the reflected ray.
This law states that angle of incidence is equal to the angle of reflection.
∠i = ∠r

Question 12.
The following diagram shows an incident ray AO and the normal ON on a plane mirror. Find the angle of incidence and angle of reflection.

Answer:
ON is perpendicular on a plane mirror
Angle of incidence ∠i (∠AON) i.e.
Angle between incident ray and normal ray = 90° – 30° = 60°
Angle of Reflection = 60°
∴ ∠i = ∠r
∴ Angle between incident and reflected ray i.e. ∠AOB
= 60 + 60 = 120°

Question 13.
State in words, how do you find the location of image of an object formed by a plane mirror.
Answer:
The location of image of a point object is as far behind the mirror as the object is in front of it.

Question 14.
Draw a ray diagram showing the formation of image of a point object by a plane mirror.
Answer:

Question 15.
The following diagram shows a point object O placed in front of a plane mirror. Take two rays from the point O and show how the image of O is formed and seen by the eye.

Answer:

Question 16.
State four characteristics of the image formed by a plane mirror.
Answer:
(i) The image formed is erect.
(ii) The image is of the same size as that of the object.
(iii) The image is laterally inverted. Right side appears to be left and left side appeared to be right.

Question 17.
How is the position of image formed by a plane mirror related to the position of the object ?
Answer:
The image formed by a plane mirror is laterally inverted, upright, of the same size and is formed far behind the mirror as the object is in front of it.

Question 18.
You are standing at a distance 2 metre from a plane mirror.
(a) What is the distance of your image from the mirror ?
(b) What is the distance between you and your image ?
Answer:
(a) Distance of image from the mirror is also 2 metre.
(b) Distance between me and my image is 4 metre.

Question 19.
What is meant by lateral inversion of an image in a plane mirror ? Explain it with the help of a diagram.
Answer:
Lateral Inversion : Inter change of sides between the object and its image is called Lateral Inversion.
Example :

It means image formed behind the mirror is as far behind the mirror as object is in front of it.
i.e. distance of M = dist of M in distance of O in front of mirror = distance of O image, and so on.

Question 20.
Wirte down the letter C and I as seen in a plane mirror.
Answer:

Question 21.
What is irregular reflection ? Give an example.
Answer:
Irregular reflection – When a beam of light falls on such a surface which is not perfectly smooth and polished such as wall, wood, paper etc. the different portions of the surface reflect light in different directions. Such a reflection of light in different directions. Such a reflection of light from an uneven surface is called the irregular or diffused reflection.

Question 22.
How do we see objects around us ?
Answer:
Objects are seen when light after striking them, returns in the same medium and reach our eyes.

Question 23.
State two uses of a plane mirror.
Answer:
It is used as :
(i) Looking glass
(ii) In periscopes.

Question 24.
Can light travel in vacuum ?
Answer:
Yes, light can travels in vacuum or air, a distance of nearly 299, 792, 458 metre (or nearly 3 x 108 metre) in one second. Thus, the speed of light in vacuum (or air) is 3 x 108 m s’1 nearly.

Question 25.
State the speed of light in (a) air, (b) glass.
Answer:
(a) Air — 3 × 10⁸
(b) Glass — 2 × 10⁸

Question 26.
State whether light slows down or speeds up in the following cases :
(a) Light going from air to glass.
(b) Light going from glass to water.
(c) Light going from water to air.
Answer:
(a) Slows down
(b) Speeds up
(c) Speeds up

Question 27.
What are the primary colours ? Name the three primary colours.
Answer:
Primary colours are the colours of light by mixing which white light is obtained. They are : (i) red (ii) green and (iii) blue.
Red + Green + Blue = White

Question 28.
What are the secondary colours ? Name the three secondary colours.
Answer:
Secondary colours are the colours of light which are obtained by mixing the two primary colours. They are (i) yellow, (ii) cyan, and (iii) magenta.

Question 29.
Fill in the blanks with the appropriate colour
(a) Blue + ………… = Cyan
(b) Red + Blue + …………. = Vhite
(c) Red + Blue = …………
(d) Green + Red = …………
Answer:
(a) Blue + Green = Cyan
(b) Red + Blue + Green = White
(e) Red + Blue = Magenta
(d) Green + Red = Yellow

Question 30.
The leaves appear green when seen in white light. Give a reason.
Answer:
Leaves appear green in white light because they reflect only the green light and absorb the light of all the other colours.

Question 31.
A rose appears red in white light. How will it appear in
(i) green light, (ii) red light ? Give a reason for your answer for each.
Answer:
(i) If a red rose is seen in green light, it appears black. The reason is that the rose absorbs the green light falling on it and reflect none.
(ii) If a red rose is seen in red light, it appears bright red. This is because the rose reflects the red light falling on it and absorbs none of it.

Question 32.
Why does a piece of paper appear white in sunlight ? How would you expect it to appear when viewed in red light?
Answer:
A piece of paper appears white in sunlight because it reflects light of all the colours. It would appear red when viewed in red light.

Question 33.
A piece of paper appears black in sunlight. What will be it£ colour when seen in red light ?
Answer:
A piece of paper appear black in sunlight. It would appear black when seen in red light because it absorbs light of all the colours.