Selina Concise ICSE Solutions for Class 8 Physics – Matter

Selina Concise Physics Class 8 ICSE Solutions – Matter

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APIusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 8 Physics Chapter 1 Matter. You can download the Selina Concise Physics ICSE Solutions for Class 8 with Free PDF download option. Selina Publishers Concise Physics for Class 8 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 8 Physics Chapter 1 Matter

•  Matter Every substance living and non-living that we see is made up of matter and MATTER “is something which has mass, occupies space and can be perceived by our senses.” e.g. Hydrogen, milk, oxygen, pen, table, water, iron, air, oil, sugar etc.
• Matter is composed of tiny particles called molecules, which are in constant motion, have spaces between them and have inter-molecular attraction.
• Every molecule can exist freely in nature and has all the properties of matter.
• A molecule is composed of ATOMS, but atom cannot exist free in nature.
•  INTER-MOLECULAR FORCE ‘The molecules of a matter exert a force of attraction on each other – The force of attraction is called INTER-
• MOLECULAR FORCE This force in solid is very strong and we cannot break a solid easily. In liquids this force is less strong and in molecules of gas it is very less. –
• FORCE OF COHESION “The inter-molecular force of attraction between the molecules of same substance is called FORCE OF COHESION.” i.e. between water and water.
• FORCE OF ADHESION “The force of attraction between the molecules of two different substances is called FORCE OF ADHESION” i.e. between glue and paper.
  Matter is composed of tiny particles and molecules of matter have spaces between them can be proved by experiment.
  Take 50 c.c of water in a measuring cylinder. Add a small quantity of salt in it. Salt gets dissolved in water and still level remains at 50 c.c. Where has salt gone?
  The salt molecules enter into spaces of water and water molecules into spaces of salt molecules. This experiment show that particles of matter are very minute and cannot be seen by naked eye and there are spaces between molecules.
  

• The molecules of matter are in constant motion can be seen by opening a bottle of perfume in a comer of room, it reaches the other parts of the room.
•  SUBLIMATION Change of solid directly into vapours on absorbing heat.
• DEPOSITION “The change of vapours directly into solid on cooling.”
• MELTING “Change of solid in liquid at fixed temperature on heating.”
• FUSION or FREEZING “Change of liquid to solid state on cooling at a fixed temperature.”
• FUSION or MELTING “Change of a solid to liquid state at a fixed temperature on absorbing heat.”
• EVAPORATION Surface phenomenon i.e. only takes place at surface “Is change of liquid to vapours.” Evaporation has cooling effect. Takes place at all temperatures.
• VAPORIZATION “Change of liquid to vapour state on heating at constant temperature.”
  It is fast process and produces hotness.

Test Yourself

A. Objective Questions

1. Write true or false for each statement

(a) The temperature of a substance remains unaffected during its change of state.
Answer: True.

(b) Ice melts at 100°C.
Answer: False. The ice melts at 0° by absorption of heat.

(c) Water at 100°C has more heat than the steam at 100°C.
Answer: False.

(d) Evaporation of a liquid causes cooling.
Answer: True.

(e) Water evaporates only at 100°C.
Answer: False.

(f) Boiling takes place at all temperatures.
Answer: False.

(g) Evaporation takes place over the entire mass of the liquid.
Answer: False.

(h) The process of a gas converting directly into solid is called vaporization.
Answer: False.
The process of a liquid converting directly into gas is called vaporization.

(i) At high altitudes water boils above 100° C.
Answer: False.

(j) The melting point of ice is 0°C.
Answer: True.

2. Fill in the blanks

(a) Evaporation takes place at all temperature.
(b) Freezing process is just reverse of melting.
(c) Sublimation is a process that involves direct conversion of a solid into its vapour on heating.
(d) The temperature at which a solid converts into a liquid is called its melting point.
(e) The smallest unit of matter that exists freely in nature is called molecule.
(f) Molecules of a substance are always in a state of motion and so they possess kinetic energy.
(g) Intermolecular space is maximum in gases less in liquids and the least in solids.
(h) Intermolecular force of attraction is maxiumum in solids, less in liquids and the least in gases.

3. Match the following

4. Select the correct alternative

(a) The inter-molecular force is maximum in

1. solids
2. gases
3. liquids
4. none of the above

(b) The inter-molecular space is maximum in

1. liquids
2. solids
3. gases
4. none of the above

(c) The molecules can move freely anywhere in

1. gases
2. liquids
3. solids
4. none of the above

(d) The molecules move only within the boundary of

1.  liquids
2. gases
3. solids
4. none of the above

(e) The temperature at which a liquid gets converted into its vapour state is called its

1. melting point
2. boiling point
3. dewpoint
4. freezing point.

(f) Rapid conversion of water into steam is an example of

1. evaporation
2. freezing
3. melting
4. vapourization.

(g) Evaporation takes place from the

1. surface of liquid
2. throughout the liquid
3. mid-portion of the liquid
4. bottom of liquid.

(h) Boiling takes place from the

1. the surface of the liquid
2. throughout the liquid
3. mid-portion of liquid
4. none of the above.

Short/Long Answer Questions

Question 1.
Define the term matter. What is it composed of ?
Answer:
Anything which occupies space and has mass is called matter. Matter is composed of tiny particles called MOLECULES.

Question 2.
State three properties of molecules of a matter.
Answer:

1. They are very small in size.
2. They have spaces between them.
3. They are in constant motion and they posses kinetic energy.

Question 3.
What do you mean by the inter-molecular spaces ? How do they vary in different states of matter ?
Answer:
INTER-MOLECULAR SPACES “The spacing between the molecules of matter is called Inter-molecular spaces.”
The inter-molecular spaces is less in solids more in liquids and still more in gases.
Explanation of inter-molecular space : Take water in a measuring cylinder say upto 80 ml. mark. Add 10 gm of salt to it. The volume in cylinder should increase. On dissolving salt we find volume remains same i.e. upto 80 ml mark. This is because there are spaces in water molecules and salt molecules occupy these spaces and volume remains the same.

Question 4.
What is meant by the inter-molecular forces of attraction ?
Answer:
How do they vary in solids, liquids and gases ?
INTER-MOLECULAR FORCES OF ATTRACTION : “The forces of attraction between the molecules of matter is called the inter-molecular force of attraction.”
This inter molecular force is maximum in solids, less in liquids and least in gases.

Question 5.
Which of the following are correct ?
Answer:
(a) Solids have definite shape and definite volume.
True.
Reason As the molecules here have negligible intermolecular distance between them and have maximum intermolecular force of attraction.
(b) Liquids have definite volume but do not have definite shape.
True.
(c) Gases have definite volume but no definite shape.
False.
Correct Gases have neither definite volume nor a definite shape.
(d) Liquids have definite shape and definite volume.
False.
Correct Liquids have a definite volume but not definite shape.

Question 6.
Discuss the three states of matter solid, liquids and gas on the basis of molecular model.
Answer:
Solids

Here the molecules are very tightly 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.
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 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.
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’hlso do not have any definite volume.

Question 7.
What do you mean by the change of state ? Write the flow chart showing the complete cycle of change of state.
Answer:
CHANGE OF STATE: “The process of change from one state(form) to another state (form) either by absorption or rejection of heat at a constant temperature is called the CHANGE OF STATE.”
COMPLETE CYCLE OF CHANGE OF STATE : On heating a solid changes to liquid and liquid on heating changes to vapours. On cooling vapours condense to LIQUID, LIQUIDS on cooling freeze to SOLIDS. Some SOLIDS on heating change to vapours. On rejection of heat vapours solidify.

This cycle can be shown by diagram

Question 8.
Differentiate between melting point and boiling point, giving atleast one example of each.
Answer:
MELTING POINT:
The temperature at which a solid starts changing into LIQUID without further increase in temperature is called MELTING POINT.” Or The constant temperature at which a solid changes into liquid.”
Example : Ice (solid) melts at Q?C into water (liquid) when heated.
BOILING POINT : “The temperature at which a LIQUID start changing in vapour without further rise in temperature.
Or
‘The constant temperature at which a LIQUID starts changing into GAS (vapours)
Example : Boiling point of water (liquid) is 100°C.

Question 9.
Describe the process of condensation and sublimation with examples.
Answer:
CONDENSATION :
“The change of vapours on cooling at fixed temperature to liquid is called condensation.”
Example: When water vapours at 100°C are cooled they change into water (liquid).
SUBLIMATION : “The process of change of solid directly into vapours on heating is called sublimation.”

Question 10.
Explain the term melting and melting point.
Answer:
Melting — The change from the solid state to the liquid state on heating at a fixed temperature is called melting.
Melting Point — It can be defined as the fixed temperature at which a solid starts changing to its liquid state.

Question 11.
Describe an experiment to demonstrate that a substance absorbs heat during melting without change in its temperature.
Answer:
MELTING POINT OF SOLID (WAX): Put some wax in a test tube. Insert a thermometer in solid wax, so that bulb of thermometer remains in wax and does not touch the sides. Clamp the test tube along with thermometer in hot bath i.e. in water contained in the beaker and set up the apparatus as shown. Note the temperature Heat the beaker over the flame of burner and record the temperature after every minute. First temperature rises and then reaches 55 °C and wax shines in the test tube. Temperature remains constant for nearly 5 minutes i.e. at 55 °C. This means Wax is melting and temperature remains constant till whole of wax is melted. Then temperature rises again every minute till it reaches
Conclusion : The temperature remains constant at 55°C while changing from solid to liquid. This means 55°C is the melting point and heat is absorbed without change in temperature. This heat is absorbed at constant temperature till whole of wax is melted.

Question 12.
Explain the terms vaporization and boiling point.
Answer:
VAPORIZATION: “Change of liquid to vapours (gas) on heating at constant temperature is called VAPORIZATION.”
When we heat a liquid temperature starts rising till it starts changing into vapours and then temperature remains constant for sometime, through we are supplying heat. This heat supplied is being used to change every molecule of liquid into vapours and temperature does not rise till the whole of liquid is changed into vapours.
BOILING POINT : “The temperature at which a liquid starts changing into vapours or gas at constant temperature is called its BOILING POINT.”

Question 13.
A liquid can change into vapour state
(a) at a fixed temperature, and
(b) at all temperatures
Name the processes involved in two cases.
Answer:
(a) is Boiling point
(b) is Evaporation.
The process involved in two’cases is vaporization or boiling.

Question 14.
Some ice is taken in a beaker and its temperature is recorded after each one minute. The observations are listed below

From the above observations what conclusion do you draw about the melting point of ice ?
Answer:
From the above observations we conclude that ice melts at 0°C during which heat is supplied but temperature does not rise shows that heat supplied is used to change every molecule of ice into water and when whole of ice is melted, temperature starts rising.

Question 15.
Describe an experiment to demonstrate that water absorbs heat during boiling at a constant temperature.
Answer:
BOILING POINT OF WATER AT CONSTANT TEMPERATURE:
Take some water in a beaker. Suspend and clamp a thermometer in beaker in water so that bulb of thermometer remains in water without touching bottom and sides of beaker. Supply heat by burner and note the temperature at room temperature (20°C nearly)

Record the temperature after evefy minute. Temperature rises and as it reaches 100°C water starts boiling. Though heat is being supplied temperature does not rise i.e. it remains constant at 100°C and bubles formed are seen. Thus, boiling point of water is 100°C and at boiling point heat supplied is absorbed by water at constant temperature. Because this heat is being used to change every molecule of water into vapours

Question 16.
State (a) the melting point of ice, and (b) the boiling point of water.
Answer:
(a) MELTING POINT OF ICE: “Is the constant temperature at which it starts (melting) changing from ice to water.”
It is 0°C for ice.
(b) BOILING POINT OF WATER : “Is that constant temperature at which water starts (BOILING) changing from water to steam (vapours)”.
It is 100°C for water.

Question 17.
What is evaporation ?
Answer:
EVAPORATION : “The change of state of a liquid to vapour at all temperatures from the surface of liquid is called evaporation.”

Question 18.
State three factors which affect the rate of evaporation of a liquid.
Answer:
Three factors on which affect the rate of evaporation of a liquid:
(i) AREA OF EXPOSED SURFACE.
(ii) TEMPERATURE OF LIQUID.
(iii) NATURE OF THE LIQUID.
(iv) PRESENCE OF HUMIDITY.

Question 19.
Wet clothes dry more quickly on a warm dry day than on a cold humid day. Explain.
Answer:
Rate of evaporation is directly proportional to temperature. Thus, rate of evaporation is higher on warm day i.e. hot day than cold day having low temperature and clothes dry soon on warm day.

Question 20.
Water in a dish evaporates fasterjhan in a bottle. Give reason.
Answer:
Rate of evaporation is more when the area of exposed surface is more. As area exposed in a dish is more, evaporation is also more.

Question 21.
Why are volatile liquids such as alcohol and spirit stored in tightly closed bottles ?
Answer:
Rate of evaporation depends on NATURE OF LIQUID i.e. more volatile liquids like ALCOHOL and SPIRIT evaporate easily, hence these are stored in tightly closed bottles to avoid their evaporation.

Question 22.
A certain quantity of water is heated from 20°C to 100°C. Its temperature is recorded after each 1 minute. The observations are:

What conclusion do you draw from the above table about the boiling point of water ? Explain.
Answer:
From the table given above we note that as thermometer shows 100°C, it becomes constant and through heat is being supplied. This means boiling point of water is 100°C and heat supplied is being used to convert every molecule of water into vapours (steam) till whole of the water gets boiled off.

Question 23.
Why is cooling produced on evaporation of a liquid ?
Answer:
For evaporation of a liquid it requires HEAT. This heat is taken from the surroundings like body or palm or fore-head or finger and its temperature falls and we feel cool.

Question 24.
Explain with an example to demonstrates that when a liquid
evaporates, it takes heat from its surroundings.
Answer:
If some spirit is poured on cotton wrapped around the bulb of a thermometer, the reading of the thermometer falls. This shows that cooling is produced when a liquid evaporates taking heat from surroundings.

Question 25.
Give two applications of evaporation.
Answer:
Two APPLICATIONS OF EVAPORATION:
(i) When we sprinkle water on the roads in summer evening, water evaporates by taking heat from the road and produces coolness in the surroundings and it becomes pleasant.
(ii) After taking a bath in summer when we come out of water, water evaporates taking heat from our body. The temperature of body falls and we feel refreshed.

Question 26.
Explain why in hot summer days water remains cool in earthen pots.
Answer:
Water seeps out through the pores in the earthen pot and it evaporates. The latent heat required for evaporation is taken, from water inside the~pot which gets cooled.

Question 27.
A patient suffering from high fever is advised to put wet clot strips on his forehead. Why ?
Answer:
Water in wet’ strips evaporates taking latent heat required for evaporation from the forehead. The temperature of forehead (body of the patient) falls and he feels relieved.

Question 28.
What do you mean by sublimation ? Explain with an example.
Answer:
SUBLIMATION : “Change of solid state of matter directly on heating to vapour state (without becoming liquid) and on cooling vapours to solid is called sublimation

Question 29.
Why does the size of naphthalene balls decrease when left open ?
Answer:
When naphthalene balls are left open, due to sublimation they change to vapours and their size decreases.

Question 30.
Describe an experiment to demonstrate the process of sublimation.
Answer:
Experiment: Take some Ammonium chloride powder in a china dish. Cover the china dish with inverted funnel and put a cotton plug in end of funnel so that vapours do not escape. Set up the apparatus as shown. Heat the dish with burner. Solid ammonium chloride changes into vapours. Which when come in contact of walls of funnel get cooled and change to solid and get deposited there.

Selina Concise Physics Class 8 ICSE Solutions – Physical Quantities and Measurement

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 8 Physics Chapter 2 Physical Quantities and Measurement. You can download the Selina Concise Physics ICSE Solutions for Class 8 with Free PDF download option. Selina Publishers Concise Physics for Class 8 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

Selina Concise physics Class 8 ICSE SolutionsChemistryBiologyMathsGeographyHistory & Civics

Selina Concise ICSE Solutions For class 8 Physics chapter 2 – Physical Quantities and Measurement

• MASS Is the quantity of matter contained in a body.
  VOLUME is the space occupied by body.
• Equal mass of IRON and cotton, iron will have less volume than cotton.
• Equal volume of Iron and cotton, the mass of iron is more than mass of cotton, because iron denser than cotton.
• DENSITY “Is ratio of mass of substance to volume of substance”
  D = M/V = KG/M³ The SI. unit of density is kg M^-3
• Density of a substance does not change with change in shape or size.
• When a substance is heated it expands and volume increases. Hence density decreases.
  Water has maximum density at 4°C i.e. density of water increases from 0°C to 4°C and decreases above 4°C.
• Volume of substance is measured by formula V = L × B × H or 4/3 or by measuring cylinder.
• Mass is measured by beam balance or spring balance.
• RELATIVE DENSITY of substance is the density compare with water i.e. How many times the substance is DENSER than water. Since density of water is 1  Gcm^-3, so density of a substance in Gcm^-3 = relative density of substance.
  S.I. unit of R.D. > has no units — since it is the ratio of same
  quantities.
• If a substance has density more than liquid it SINKS in the liquid and if the density of substance is LESS than liquid it floats on liquid.
• BUOYANT FORCE “The force exerted by liquid acting vertically
  upward on a body and is equal to the weight of liquid displaced by its immersed part.”
• Weight of body Acting vertically downward. This force has the tendency to sink the body.
•  LAW OF FLOATATION “When a body floats in a liquid, the weight of the liquid displaced by its immersed part is equal to the total weight of the body.” While floating
  wt. of floating body W=wt. of liquid displaced by its immersed part FB i.e. Apparent wt. of floating body is zero.
  

Density of body is greater than density of liquid. The body sinks.

Density of body is equal to the density of liquid. The body float where ever it is left in liquid.

Density of body is less than density of liquid. The body rises to the surface and floats.

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) Equal volumes of the two different substances have equal masses.
Answer False.
Equal volumes of the two different substances have different masses.

(b) The density of a piece of brass will change by changing its size or shape.
Answer False.

(c) The density of a liquid decreases with increase in its temperature.
Answer True.

(d) Relative density of water is 1.0.
Answer True.

(e) Relative density of a substance is expressed in g cm-3.
Answer False.
Relative density of a substance has no units.

(f) When a body is immersed in a liquid, the buoyant force experienced by the body is equal to the volume of the liquid displaced by it.
Answer False.
The buoyant force is equal to the weight of the liquid displaced by the immersed part of body.

(g) A body experiences the same buoyant force while floating in watr or alcohol.
Answer True.

(h) A body experiences the same buoyant force when it floats or sinks in water.
Answer False.

(i) A body floats in a liquid when its weight becomes equal to the weight of the liquid displaced by its submerged part. .
Answer True.

(j) A body while floating, sinks deeper in a liquid of low density than in a liquid of high density.
Answer True.

2. Fill in the blanks

(a) 1 kg is the mass of 1000 ml of water at 4°C.
(b) Mass = density x volume.
(c) The S.I. unit of density is Kg m^-3
(d) Density of water is 1000 Kg m^-3.
(e) 1 g cm^-3 = 1000 Kg m^-3.
(f) The density of a body which sinks in water is more than 1000 Kg m^-3.
(g) Abody sinks in a liquid A, butt floats in a liquid B. The density of liquid A is less than the density of liquid B.
(h) A body X sinks in water, but a body Y floats on water. The density of the body X is more than the density of body Y.
(i) The buoyant force experienced by a body when floating in salt¬water is equal to or same that of when floating in pure water.
(j) The weight of a body floating in a liquid is zero.

3. Match the following

4. Select the correct alternative 

(a) The correct relation is

1. Density = Mass x Volume
2. Mass = Density x Volume
3.  Volume = Density x Mass
4. Density = Mass + Volume

(b) The relative density of alcohol is 0.8. Its density is

1.  0.8
2. 800 kg nr3
3.  800 g cm-3
4. 0.8 kg m-3

(c) A block of wood of density 0.8 g cm-3 has a volume of 60 cm3. The mass of block is

1.  60.8 g
2.  75 g
3. 48 g
4. 0.013 g

(d) The density of aluminium is 2.7 g cm-3 and that of brass 8.4 g cm’3. The correct statement is

1.  Equal masses of aluminium and brass have equal volumes
2. The mass of a certain volume of brass is more than the mass of equal volume of aluminium.
3.  The volume of a certain mass of brass is more than the volume of equal mass of aluminium.
4.  Equal volumes of aluminium and brass have equal masses.

(e) A density bottle has a marking 25 mL on it. It means that:

1.  the mass of density bottle is 25 g
2. the density bottle will store 25 ml of any liquid in it
3.  the density bottle will store 25 ml of water, but more volume of liquid denser than water.
4.  the density bottle will store 25 ml of water, but more volume of a liquid lighter than water.

(f) The correct statement is

1.  The buoyant force on a body is equal to the volume of the liquid displaced by it ‘
2. The buoyant force on a body is equal to the volume of the body
3. The buoyant force on a body is equal to the weight of the liquid displaced by it
4.  The buoyant force on a body is always equal to the weight of the body.

(g) A piece of wood floats on water. The buoyant force on wood will be

1.  zero
2.  more than the weight of the wood piece
3. equal to the weight of the wood piece
4. less than the weight of the wood piece.

(h) The weight of a body is more than the buoyant force experienced by it, due to a liquid. The body will

1. sink
2.  float with its some part outside the liquid
3.  float just below the surface of liquid
4. float with whole of its volume above the surface of liquid.

B. Short/Long Ans Questions 

Question 1.
Define the term density of a substance.
Answer:
Density of a substance is defined as “Mass per Unit volume”.

Question 2.
Name the S.I. unit of density. How is it related to g Cm^-3 ?
Answer:
S.I. unit of density is kg M^-3 In C.GS. system unit of mass is g and unit of volume is Cm³, so CGS unit of density is g Cm^-3 (gram per cubic centimetre)
Relationship between S.I. and C.GS. units

Question 3.
The density of brass is 8.4 g cm-3. What do you mean by this statement ?
Answer:
This statement meAns one cubic centimetre volume of brass has mass of 8.4 g.

Question 4.
Arrange the following substances in order of their increasing density:
Iron, Cork, Brass, Water, Mercury.
Answer:
Cork, Water, Iron, Brass, Mercury.

Question 5.
How does the density of a liquid (or gas) vary with temperature?
Answer:
Most of the liquids increase in volume with increase in temperature, but water shows anomalous behaviour. Water has maximum volume at 4°C and maximum density at 4°C.
Actually, when volume increases density decreases and when volume decreases the density increases.
But water when cooled from a high temperature, contracts upto 4°C because volume decreases and expands when cooled further below 4°C and hence density of water increases when it is cooled upto 4°C while decreases when cooled further below 4°C. In other words, the density of water is maximum at 4°C equal to 1 g Cm^-3 or lOOO kg m^-3.

Question 6.
A given quantity of a liquid is heated. Which of the following quantity will vary and how ?
(a) mass, (b) volume and (c) density
Answer:
When a given quantity of liquid is heated
(a) Mass : does not change.
(b) Volume: changes and increases with rise in temperature.
(c) Density : Changes and decreases.
Density = Mass / volume

Question 7.
Describe an experiment to determine the density of the material of a coin.
Answer:
Density = Mass / volume
To find the density of the material of a coin, we need to find its (i) mass—by common beam balance and (ii) Its volume by measuring cylinder.
Measure the mass of coin.
EXPERIMENT – Let the mass of coin shown by beam balance = M (gram) = 50 g (ray)
Measure the vol. of coin.
Initial volume of water = V₁ = 40 ml (say)
Final volume of water
When coin is added in the cylinder=V₂ = 50 ml (say)
Then vol. of coin = V₂ – V₁ = 50 – 40 = 10 ml

Question 8.
Describe an experiment to determine the density of a liquid.
Answer:
To determine the density of a liquid D = M / V
We need to find (i) the vol. of liquid say milk, (ii) mass of liquid.
EXPERIMENT:
(i) To find the mass of milk:
wt. of empty 100 c.c beaker = M₁ g = 70 g (say)
Fill the beaker (half) with milk and weigh again=M₂ g = 116 g (say)
(ii) To find the vol. of milk:
TrAnsfer this milk into measuring cylinder and note the volume V = 40 c.c (say)

Question 9.
What is a density bottle ? How is it used to find the density of a liquid ?
Answer:
DENSITY bottle is a small glass bottle having a glass stopper at its neck. The bottle can store a fixed volume of a liquid. Generally the volume of bottle is 25 ml or 50 ml. Stopper has a narrow hole through it. When bottle is filled with liquid and stopper is inserted, THE EXCESS LIQUID RISES THROUGH THE HOLE and drains out. Thus the bottle will contain the same volume of liquid each time when it is filled. It is used to determine the density of a liquid.

Question 10.
Define the term relative density of a substance.
Answer:
RELATIVE DENSITY: “is the ratio of density of a substance to the density of water at 4° C.”
Or
RELATIVE DENSITY “is theratio of mass of the substance to the mass of an equal volume of water at 4° C.”

Question 11.
What is the unit of relative density ?
Answer:
UNIT OF RELATIVE DENSITY: No units since it is a pure ratio.

Question 12.
Distinguish between density and relative density.
Answer:

Question 13.
Explain the meaning of the statement ‘relative density of aluminium is 2.7’ ?
Answer:
The statement ‘Relative density of aluminium is 2.7’ meAns .
A piece of aluminium of any volume has mass 2.7 times that of an equal volume of water.
i.e. Aluminium is 2.7 times heavier than water.

Question 14.
How does the density of a body and that of a liquid determine whether the body will float or sink into that liquid ?
Answer:
If the density of a body is LESS than the density of LIQUID, the body will FLOAT on the surface of liquid.
If the density of a body is MORE than the density of liquid, the body will SINK in a liquid.

Question 15.
A cork piece floats on water surface while an iron nail sinks in it. Explain the reason.
Answer:
CORK floats on water meAns density of cork is LESS than density of water.
IRON nail: Sinks in water meAns density of iron nail is MORE than density of water.

Question 16.
Which of the following will sink or float on water ? (Densityof water = 1 g Cm^-3)
(a) body A having density 500 kg m^-3
(b) body B having density 2520 kg m^-3
(c) body C having density 1100 kg m^-3
(d) body D having density 0.85 g m^-3
Answer:
Density of water = 1 g Cm^-3
(a) Density of body A = 500 kg m^-3 = 500 × = 0.5 = 0.5 g Cm^-3
Density of body A ¡s less than density of water hence A will float on water
(b) Density of body B = 2520 kg m^-3 = 2520 × 1/1000 = 2.52 g Cm^-3
Density of body B is more than density of water and hence B will SiNK in water
(c) Density of body C = 1100kg m^-3 = 1100 × 1/1000 = 1.1 g Cm^-3
is greater than water.
Hence, body C will sink in water.
(d) Density of body D = 0.85 g Cm^-3 < 1.0 g Cm^-3
Density of body D is less than the density of water hence body D will FLOAT on water

Question 17.
What is the iaw of floatation ?
Answer:
When a body floats in a liquid, the weight of the liquid displaced by its immersed part is equal to the total weight of the body. This is the law of floatation, i.e. while floating. Weight of the floating body = Weight of the liquid displaced by its immersed part.

Question 18.
The density of water is 1.0 g Cm^-3. The density of iron is 7.8 × 10″3 g Cm^-3. The density of mercury is 13.6 g Cm^-3.
Ans the following:
(a) Will a piece of iron float or sink in water ?
(b) Will a piece of iron float or sink in mercury ?
Answer:
Density of water 1.0 g Cm^-3
(a) Density of piece of iron = 7.8 × 10^-3 g Cm^-3

∴ Density of piece of iron is LESS than density of water.
Hence, piece of iron will FLOAT in water.
(b) Density of piece of iron = 7.8 × 10^-3
Density of mercury is 13.6 × 10^-3 g Cm^-3
Since 7.8 × 10^-3 < 13.6 × 10^-3
∴ Density of piece of iron is LESS than density of mercury
∴ Piece of iron will FLOAT in mercury

Question 19.
The diagram given below show a body floating in three different liquids. A, B and C at different levels.
(a) In which liquid does the body experience the greatest buoyant force ?
(b) Which liquid has the least density ?
(c) Which liquid has the highest density ?

Answer:
(a) Buoyant force is same in each case as the wt. of body is same in each case and Buoyant force is equal to the weight of liquid displaced by the immersed part of body which balances the wt. of body.
(b) The liquid A has the least density as body immerses the maximum.
(c) Liquid C has the highest density as the body immerses the least.

Question 20.
For a floating body, how is its weight related to the buoyant force ?
Answer:
When a body floats in a liquid. The weight of the liquid displaced by its immersed part is equal to the total weight of the body.

Question 21.
Why does a piece of ice float on water ?
Answer:
FLOATATION OF ICE ON WATER : Density of 0.9 g Cm^-3 is less than density of water 1 g Cm^-3. Hence, ice floats on water.

Question 22.
Explain why an iron needle sinks in water, but a ship made of iron floats on water.
Answer:
Density of iron is more than density of water, ∴ weight of iron nail is more than wt. of water displaced by it and nail SINKS. While shape of iron ship is made in such a way that it displaces MORE WEIGHT OF WATER than its own weight. Secondly the ship is HOLLOW and THE EMPTY SPACE contains AIR which makes the AVERAGE DENSITY OF SHIP LESS THAN THAT OF WATER and hence ship floats on water.

Question 23.
It is easier to swim in sea water than in river water. Explain the reason.
Answer:
Density of sea water is greater than density of river water, [because of impurities]
(i) In each case the weight of water displaced will be equal to the weight of the man.
∴ Ratio of weight of sea water and river water displaced by man is 1: 1.
(ii) With smaller portion of man’s body submerged in sea water, the wt. of sea water displaced is equal to the total weight of body. While to displace the same weight of river water, a larger portion of the body will have to be submerged ¡n water.
∴ It is easier for man to swim in sea water.

Question 24.
Icebergs floating on sea water are dangerous for ships. Explain the reason.
Answer:
ICEBERGS are very dangerous for ships as ICEBERGS are huge masses of ice floating in sea [density of ice being 0.917 g Cm^-3]
with about 9/10 portion below water and only 1/10 portion of it above surface of water.

Question 25.
Explain why it is easier to lift a stone under water than in air.
Answer:
In water, the stone experience a buoyant force which counter balances the weight of the stone acting downward and this makes the stone lighter and thus easier to lift the stone in water.

Question 26.
What is a submarine ? How can it be made to’dive in water and come to the surface of water.
Answer:
SUBMARINE: Submarine is a water-tight boat which can travel under water like a ship. It is providgd with water tanks. When submarine is to dive, water is filled in water tanks and it is made heavier and average density of submarine becomes greater than the density of sea water and it sinks. To make the submarine rise to the surface of water, water tanks are emptied and average density.of submarine becomes less than the density of sea water and it rises to surface of water.

While submarine is underwater soldiers can see the enemy activities through periscope.

Question 27.
A balloon filled with hydrogen rises in air. Explain the reason.
Answer:
A balloon filled with hydrogen rises to a certain height as it displaces more wt. of air than wt. of balloon but as it rises higher density of air DECREASES there and upthrust becomes less and ultimately upthrust becomes equal to the weight of balloon and balloon stops rising further.

C. Numericals

Question 1.
The density of air is 1.28 g/Iitre. Express it in:
(a) g cm³ (b) kg m
Answer:
(a) The density of air is I .28g/litre

Question 2.
The dimensions of a hail are 10 m × 7 m × 5 m. If the density of air is 1.11 kg m^-3, find the mãss of air in the hail.
Answer:
The dimensions of hall 10m × 7m × 5m
i.e. V350 m³
Density of air(D)= 1.11 kg m^-3
M = V × D 350 ×  1.11 =388.5 kg

Question 3.
The density of aluminium is 2.7 g cm3. Express it in kg m^-3
Answer:
Density of aluminium = 2.7 g/Cm³

Question 4.
The density of alcohol is 600 kg m^-3. Express it in g Cm^-3.
Answer:
Density of alcohol is = 600 kg/m^-3

Question 5.
A piece of zinc of mass 438.6 g has a volume of 86 Cm³. Calculate the density of zinc.
Answer:

Mass of Zinc (M) = 438.6 g
Volume V = 86 Cm³
Density (D) = ?

Question 6.
A piece of wood of mass 150 g has a volume of 200 Cm³. Find the density of wood ¡n
(a) C.GS. unit, (b) S.l. unit
Answer:
(a) Mass of wood (M) = 150 g
Volume of wood (V) = 200 Cm³
Density (D) =?

(b) In S.I. system = 0.75 × 1000 750 kg/ m³

Question 7.
Calculate the volume of wood of mass 6000 kg if the density of wood is 0.8 g Cm^-3
Answer:
Volume of wood (V) = ?
Mass of wood (M) = 6000 kg
Density of wood D = 0.8 g/ Cm³
D=O.8g/Cm³=o.8 × IOOO = 800kg /m³

Question 8.
Calculate the density of solid from the following data :
(a) Mass of solid = 72 g
(b) Initial volume of water in measuring cylinder = 24 ml
(c) Final volume of water when solid is completely immersed in water = 42 ml
Answer:
Mass of solid (M) = 72 g
Intial volume of water V₁ = 24 ml
Final volume of water V₂ = 42 ml
Volume of solid (V) = V₂ – V₁ = 42 – 24 = 18 Cm³
Density of solid (D) = ?

Question 9.
The mass of an empty density bottle is 21.8 g, when filled completely with water is 41.8 g and when filled completely with liquid it is 40.6 g. Find :
(a) the volume of density bottle
(b) the relative density of liquid
Answer:
Density of water is 1 g Cm³
∴ Volume of density bottle = weight of water in grams completely filling the bottle
(a) Volume of density bottle:
Mass of empty density bottle = M₁ =21.8 g
Mass of bottle + water = M₂  41.8 g
∴ Mass of water completely fih1ig the density bottle = M₂ — M₁
=41.8 —21.8
20g
But 1 g of water has volume = 1 cc
∴ Volume of bottle (density bottle) = volume of water =20 c.c. =20 ml
(b) The relative density of liquid:
Mass of 20 c.c. of liquid = (mass of density bottle + mass of 20 c.c of liquid- mass of density bottle)
= 40.6—21.8
= 18.8 g
Mass of 20 C.C of water = 20g
Relative density of liquid

Question 10.
From the following observations, calculate the density and relative density of a brine solution. Mass of empty density
bottle = 22 g
Mass of bottle + water = 50 g
Mass of bottle + brine solution = 54 g

Answer:
Mass of empty bottle, M₁ = 22 g
Mass of bottle + water, M₂ =50 g
Mass of bottle + brine solution, M₃ =54 g
Mass of water = M₂ — M₁ =50—22=28 g
Mass of brine solution = M₃ — M₁ 54—22 = 32 g
Density of brine solution = Mass of brine solution / Mass of water

Question 11.
The mass of an empty density bottlfe is 30 g, it is 75 g when filled completely with water and 65 g when filled completely with a liquid. Find :
(a) volume of density bottle,
(b) density of liquid, and
(c) relative density of liquid.
Answer:
Mass of empty density bottle (M₁) =30 g
Mass of bottle + Water (M₂) 75 g
Mass of liquid + Liquidx (M₃)= 65 g
Mass ofwater=M₂—M₁=75—30=45 g
(a) Volume of density bottle = Mass of water 45 g
(b) Density of Iiquid x = ?

(c) Mass of water in the density bottle =75 — 30 = 45 g
∴ Volume of water in density bottle = 45 cc
and mass of equal volume of liquid in density bottle 65—30 = 35g

Selina Concise Physics Class 8 ICSE Solutions – Sound

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 8 Physics Chapter 7 Sound. You can download the Selina Concise Physics ICSE Solutions for Class 8 with Free PDF download option. Selina Publishers Concise Physics for Class 8 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 8 Physics Chapter 7 Sound

• SOUND: “Is energy which produces in us the sensation of hearing.” It is produced by vibration of body.
• Sound needs a medium for its propagation. Sound cannot travel in vacuum.
• Speed of sound is maximum in solids. 5000 ms^-1 in steel, in water 1500 ms^-1 and in air it is least 330 ms^-1 nearly.
• When a body vibrates, the particles of medium also start vibrating and K.E. of particles changes into potential energy and P.E. into
  K.E. This is why sound in energy.
•  Sound travels in a medium in the form of wave.
  
• Longitudinal wave : When the particles of medium move in the direction of motion of wave by forming compression and rarefaction.
  
• AMPLITUDE : “The maximum displacement of the particle of medium on either side of mean position.”
  
• TIME PERIOD: “The time taken by a particle of medium to complete its one vibration” “t”
•  FREQUENCY: “The number of vibrations made by a particle of
  the medium in one second. ƒ measured in Hertz (Hz)
•  FREQUENCY  ƒ = 1/ t or t = 1 / ƒ
•  WAVE LENGTH: “The distance travelled by the wave in one one time period of vibration of particle of medium.”
  Or
  “The distance between two consecutive compressions or between two consecutive rarefactions.” It is denoted by ‘ λ ’ and S.I. unit of wave length is metre (m).
  
•  CHARACTERISTICS OF SOUND :
  (i) Loudness.
  (ii) Pitch (or shrillness).
  (iii) Quality (or timbre or wave form).
  LOUDNESS : is the characteristic of sound by virtue of which a loud sound can be distinguished from a faint sound, both having same frequency and same wave form.
•  It depend on: (i) Amplitude of wave (ii) Surface area of vibrating body (ii) Distance from the source of sound (iv) Sensitivity of listener: Unit of loudness is (dB) decibel.
•  PITCH: It depends on number of vibrations per second or frequency : more frequency is high pitch shrilled sound and low frequency is flat sound.
•  QUALITY: is the characteristic which distinguishes two sounds’of the same pitch and same loudness. It depends on wave form.

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) When sound propagates in air, it does not carry energy with it.
Answer. False.

(b) In a longitudinal wave, compression and rarefaction are formed.
Answer. True.

(c) The distance from one compression to nearest rarefaction is called wavelength.
Answer. False.

(d) The frequency is measured in second.
Answer. False.

(e) The quality of a sound depends on the amplitude of wave.
Answer. False.

(f) The pitch of sound depends on frequency.
Answer. True.

(g) Decibel is the unit of pitch of a sound.
Answer. False.

2. Fill in the blanks

(a) The time period of a wave is 2 s. Its frequency is 0.5 S^-1.
(b) The pitch of a stringed instrument is increased by increasing tension in string.
(c) The pitch of a flute is decreased by increasing length of air column.
(d) Smaller the membrane, higher is the pitch.
(e) If a drum is beaten hard, its loudness increases.
(f) A tuning fork produces sound of single frequency.

3. Match the following

4. Select the correct alternative

(a) Sound can not travel in

1.  solid
2.  liquid
3.  gas
4.  vacuum

(b) When sound travels in form of a wave

1. the particles of medium move from the source to the listener
2.  the particles of medium remains stationary
3.  the particles of medium start vibrating up and down
4.  the particles of medium transfer energy without leaving their mean positions.

(c) The safe limit of loudness of audible sound is

1.  0 to 80 dB
2.  above 80 dB
3.  120 dB
4.  above 120 dB

(d) The unit of loudness is

1.  cm
2.  second
3.  hertz
4.  decibel

(e) In a piano, pitch is decreased by

1.  using thicker string
2.  increasing tension
3.  reducing length of string
4.  striking it hard Ans.

Selina Concise Physics Class 8 ICSE Solutions – Electricity

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 8 Physics Chapter 8 Electricity. You can download the Selina Concise Physics ICSE Solutions for Class 8 with Free PDF download option. Selina Publishers Concise Physics for Class 8 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 8 Physics Chapter 8  Electricity

•  ELECTRICITY: “Is the rate of flow of electrons”. i = \(\overset { q }{ \underset { t }{ – } } \)
•  To keep electrons move, potential difference is needed. This is done by a cell or battery.
•  Potential difference “is the amount of work done in moving a unit positive charge from one point to other.”
  Potential difference V = Work done (W) / charge moved (Q)
  Or W = QV but charge = it
  Hence, W = VIt or Electrical energy = VIt
•  Power is “Rate of doing work”
  P = W/t = VIt/t = VI
  Power is measured in watt or J S^-1
  1 Watt = 1 Volt × 1 Ampere
• S.I. unit of charge is coulomb (C).
  S.I. unit of current is Ampere (A).
  S.I. unit of P.D. is volt.
  S.I. unit of electrical energy is Joule (J) and of power is watt (W)
  1 kWh-3600000J = 3.6 x 10⁶ J
•  ELECTRIC power is generated at the GENERATING STATION at 11000 volt, or 11 kV as these stations are at very far off place from areas where it is to be used. The voltage (A.C.) is of 50 HZ frequency.
• AT GRID SUB-STATION this alternating current (A.C) voltage is stepped up from 11 kV to 132 kV to minise the loss of energy in transmission line wires.
• At MAIN-SUB-STATION this voltage is stepped down from 132 kV to 33 kV and transmitted to city SUB-STATION.
•  At CITY SUB STATION, it is further stepped down from 33 kV to 220 V for supply to hourses for consumers.
•  Colour coding: Live wire — Red or Brown
  Neutral—Black or light blue
  Earth wire — Green or yellow
•  1 kWh = 1 unit: Power Rating on an appliance 100 W – 220 V means the appliance when worked on a 220 V will consume 100 W electricity power
• OVER LOADING: is the condition of Electric circuit, when it draws more current than it is designed for i. e. when a number of appliances are switched on at a time i.e. geyser, A.C. Electric motor etc. or a large number of plugs are put in the same socket.
•  EARTHING: is done in a house near the kWh meter. Earthing is a safety device which puts the appliance at zero potential.
•  SHORT CIRCUITING: If the insulation on the wire of cable used f in the wiring (or used with an appliance) breaks. The LIVE WIRE
  COMES IN CONTACT WITH THE NEUTRAL WIRE, this result in SHORT CIRCUITING
•  FUSE: “Is a device used to limit the current in an electric circuit”. The use of fuse protects the appliance in circuit from being damaged Fuse is always connected in live wire. A fuse wire should have
  (i) High resistance
  (ii) Low melting point.
  These days miniature circuit breakers (MCB) are used. It is AUTOMATIC breaker, when current flowing excess.
•  Appliances in a house are connected in parallel.

Test yourself

A. Objective Questions

1. Write true or false for each statement:

(a) A fuse wire has a high melting point.
Answer. False.

(b) Flow of protons constitutes electric current.
Answer. False.

(c) Silver is an insulator of electricity.
Answer. False.

(d) S.I. unit and commercial unit of electrical energy are same.
Answer. False.

(e) Overloading of electric current in circuits can lead to short circuiting.
Answer. True.

(f) Our body can pass electricity through it.
Answer. True.

(g) All metals are insulators of electricity.
Answer. False.

(h) The earth wire protects us from an electric shock.
Answer. True.

(i) A switch should not be touched with wet hands.
Answer. True.

(j) AH electrical appliances in a household circuit work at the same voltage.
Answer. True.

(k) In a cable, the green wire is the live wire.
Answer. False.

(l) A fuse is connected to the live wire.
Answer. True.

(m) A switch is connected to the neutral wire.
Answer. False.

2. Fill in the blanks

(a) The unit in which we pay the cost of electricity is kWh.
(b) The electrical energy consumed in a house is measured by kWh meter.
(c) In a household electrical circuit, the appliance are connected in parallel with the mains.
(d) A switch is connected to the live wire.
(e) The red colour insulated wire in a cable is the live wire.
(f) One kilowatt hour is equal to 3.6 x 106 joule.
(g) A fuse wire should have low melting point.

3. Match the following

4. Select the correct alternative

(a) All wires used in electric circuits should be covered with

1.  colouring material
2.  conducting material
3.  an insulating material
4.  none of the above

(b) Electric work done per unit time is

1.  electrical energy
2.  electric current
3.  electric voltage
4.  electrical power

(c) One kilowatt ¡s equal to

1.  100 watt
2.  1000 watt
3.  10 watt
4.  none of these

(d) Fuse wire is an alloy of

1.  tin-lead
2.  copper-lead
3.  tin-copper
4.  lead-silver

(e) A fuse wire should have

1.  a low melting point
2.  high melting point
3.  very high melting point
4.  none of the above

(f) When switch of an electric appliance is put off, it disconnects

1.  the live wire
2.  the neutral wire
3.  the earth wire
4.  the live and the neutral wire

(g) The purpose of an electric meter in a house is

1.  to give the cost of electricity directly
2.  to give the consumption of electrical energy
3.  to safeguard the circuit from short circuiting
4.  to put on or off the mains.

(h) If out of the two lighted bulbs in a room, one bulb suddenly fuses, then

1.  other bulb will glow more
2.  other bulb will glow less
3.  other bulb will also fuse
4.  other bulb will remain lighted unaffected.

Selina Concise Physics Class 8 ICSE Solutions – Light Energy

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 8 Physics Chapter 5 Light Energy. You can download the Selina Concise Physics ICSE Solutions for Class 8 with Free PDF download option. Selina Publishers Concise Physics for Class 8 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 8 Physics Chapter 5 Light Energy

• Light is a form of energy which produces in us the sensation of sight i.e. we can see objects only when light falls on them and then reflected into our eye.
•  Velocity of light in air or in vacuum is 300000 km per second.
  Or
  3 x 10⁸ ms^-1
• As light passes into different mediums its speed changes and depends upon the density of medium i.e. it decreases with increase in density i.e. it is 2.25 × 10⁸ m/s in water and 2 x 10⁸ ms^-1 in glass as water is
  denser than air ( \(_{ w }^{ a }{ \mu }\) = 1.33 ) and glass is still optically denser than water
  ( \(_{ g }^{ a }{ \mu }\) =1.5 ) i.e. slower in water and still slower in glass.
• Light travels in a straight line.
• As light travels from one transparent medium to other transparent medium and falls oblique at another medium, its path changes and this change in path is called REFRACTION OF LIGHT.
•  When ray of light travels from RARER (less-denser) to DENSER medium, it bends TOWARD the normal AND when it travels from a DENSER to a RARER medium it bends away from NORMAL
•  ANGLE of INCIDENCE : “The angle which incident ray makes with normal”. “∠i”
•  ANGLE OF REFRACTION: “The angle which refracted ray makes with normal” “ ∠r ”
  ∠i is not equal to ∠r
•  LAWS OF REFRACTION or SNELL’S LAWS OF REFRACTION:
  (i) Incident ray, normal at the point of incidence and Refracted ray all lie in the same plane.
  (ii) Ratio of sine of angle of incidence to the sine of angle of refraction is constant.
• 
•  EFFECTS OF REFRACTION :
  (i) A coin placed in water appears to be raised.
  (ii) Swimming pool seen from above appears SHADOW.
  (iii) A pencil in water appears to be bent.
  (iv) MIRAGE in desert, EARLY Sunrise, LATE SUN set are all due to REFRACTION of light.
• White light is a band of seven colours-VIBGYOR. Speed of all colours of the white light in AIR or VACUUM is same, but different different transparent mediums.
•  In glass or water Speed of VIOLET colour is MINIMUM and speed of RED light is MAXIMUM
• Refractive index of medium is minimum for VIOLET light and R.I. of medium is maximum for red light.
• DISPERSION: “The splitting (breaking) of white light into seven colours is called DISPERSION OF LIGHT.
•  CAUSE OF DISPERSION: Speed of different colours is different in glass or water and different colours get separated from each other on refraction at second surface of glass prism.

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) Water is optically denser than glass.
Answer. False.
Water is optically denser than air.

(b) A ray of light when passes from glass to air, bends towards the normal.
Answer. False.

(c) The speed of light is more in glass than in water.
Answer. False.

(d) The depth of a pond when seen from above appears to be less.
Answer. True.

(e) Light travels at a lower speed in water than in air.
Answer. True.

(f) Light travels in the same straight line path while passing through different media.
Answer. False.

(g) The angle formed between the normal and the refracted ray is known as the angle of incidence.
Answer. False.

(h) At the point of incidence, a line drawn at right angles to the surface, separating the two media, is called the normal.
Answer. True.

(i) Image is formed by a mirror due to refraction of light.
Answer. False.

(j) Rays of light incident parallel to the principal axis pass through the focus after reflection from a concave mirror.
Answer. True.

(k) A convex mirror is used as a shaving mirror.
Answer. False.

(l) The focal length of a convex mirror is equal to its radius of curvature.
Answer. False.

(m) A concave mirror converges the light-rays, but a convex mirror diverges them.
Answer. True.

(n) A virtual image formed by a spherical mirror is always erect and situated behind the mirror.
Answer. True.

2. Fill in the blanks

(a) Water is optically denser than air.
(b) Air is optically rarer than glass.
(c) When a ray of light travels from water to air, it bends away from the normal.
(d) When a ray of light travels from air to glass, it bends towards the normal.
(e) When white light passes through a prism, it disperses
(f) The splitting of white light into its constituent colours is called dispersion.
(g) A concave mirror is obtained on silvering the outer surface of a part of a hollow glass sphere.
(h) Radius of curvature of a spherical mirror is two times its focal length.
(i) The angle of incidence for a ray of light passing through the centre of curvature of a spherical mirror is 0°
(j) A convex mirror always forms a virtual image.
(k) A concave mirror forms a virtual image for an object placed between pole and focus.

3. Match the following

4. Select the correct alternative

(a) The speed of light in air or vacuum is

1. 3 × 10⁸ M s^-1
2.  2.25 × 10⁸ m s^-1
   
3.  332 ms^-1
4.  2.0 × 10⁸ ms^-1

(b) A ray of light moving from an optically rarer to a denser medium

1.  bends away from the normal
2.  bends towards the normal
3.  remains undeviated
4.  none of the above

(c) The angle between the normal and refracted ray is called

1.  angle of deviation
2.  angle of incidence
3.  angle of refraction
4.  angle of emergence.

(d) The property of splitting of white light into its seven constituent colours is known as

1.  rectilinear propagation
2.  refraction
3.  reflection
4.  dispersion

(e) The seven colours in the spectrum of sunlight in order, are represented as :

1.  VIBGYOR
2.  VIGYBOR
3.  BIVGYOR
4.  RYOBIVG

(f) A ray of light passing through centre of curvature of a spherical mirror, after reflection

1. passes through the focus
2.  passes through the pole
3.  becomes parallel to the principal axis
4.  retraces its own path.

(g) If the radius of curvature of a concave mirror is 20 cm, its focal length is:

1.  10 cm
2.  20 cm
3.  40 cm
4.  80 cm

(h) The image formed by a convex mirror is

1.  erect and diminished
2.  erect and enlarged
3.  inverted and diminished
4.  inverted and enlarged.

(i) The image formed by a concave mirror is of the same size as the object, if the object is placed

1. at the focus
2. between the pole and focus
3.  between the focus and centre of curvature
4.  at the centre of curvature.

(j) A convex mirror is used

1.  as a shaving mirror
2.  as a head mirror by a dentist
3.  as a rear view mirror by a driver
4.  as a reflector in torch.