Reflection Of Light

Which Type of Image is Formed by a Plane Mirror?

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Which Type of Image is Formed by a Plane Mirror?

Image Reflection by a Plane Mirror

When you look into a plane mirror, you will see an image of yourself which has the following characteristics:

  1. Your image is upright.
  2. Your image is the same size as you are.
  3. Your image is at the same distance as you are from the mirror.
    (Object distance = Image distance)
  4. Your left and right sides are interchanged in your image. So, your left hand becomes the right hand of your image. When this happens, your image is said to be laterally inverted.
  5. Your image is behind the mirror and cannot be seen on the screen. Your image is known as a virtual image.

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Ray Diagrams for Plane Mirrors

The characteristics and the position of the image formed by a plane mirror can be determined by drawing a ray diagram.
The following are the steps to draw a ray diagram when viewing an object in a plane mirror.

  • The object, O (the candle) is placed in front of a mirror, M.
    Ray Diagrams for Plane Mirrors
  • The position of its image, I is located. The image distance is equal to the object distance, OM = IM. The image size is same as the object size. The line joining the object, O and the image, I is perpendicular to the mirror, M.
    Ray Diagrams for Plane Mirrors 1
  • The reflected rays are drawn as if they are from the image, I. The image is virtual. Therefore, the rays behind the mirror do not exist. They are virtual rays and are represented by dotted lines. The continuous lines from the mirror to the eye indicate the reflected rays.
    Ray Diagrams for Plane Mirrors 2
  • The incident rays are drawn from the object, O to the mirror, M. Lines joining the object to the positions of the reflected rays on the mirror represent the incident rays.
    Ray Diagrams for Plane Mirrors 3

Image is Formed by a Plane Mirror Example Problems with Solutions

  1. A woman of height 1.5 m stands 3 m in front of a plane mirror as shown in Figure.
    Plane Mirror Example Problems 1
    (a) What is the height of her image?
    (b) How far is she from her image?
    (c) If she walks 2 m towards the mirror, how far is she from her image now?
    Solution:
    (a) Height of image = 1.5 m
    (b) Distance from image
    = 3 + 3
    = 6 m
    (c) Distance from mirror
    = 3 – 2
    = 1 m
    Distance from image
    = 1 + 1
    = 2 m
  2. The height of Dayah is 160 cm. She stands facing a mirror mounted on a wall at a distance of 160 cm. The distance between her eyes and the floor is 150 cm and she could just see her feet at the bottom edge of the mirror.
    Plane Mirror Example Problems 2
    (a) What is the distance between Dayah and her image in the mirror?
    (b) Draw light rays to show how she could see her head and her feet.
    (c) (i) How high above the floor is the bottom edge of the mirror?
    (ii) What is the minimum height of the top edge of the mirror from the floor if she is able to see the top of her head?
    Solution:
    Plane Mirror Example Problems 3
  3. Diagram shows a student standing 5 m in front of a mirror.
    Plane Mirror Example Problems 4
    If the mirror is moved towards the student by a distance of 1 m, what is the distance between the student and his image?
    Solution:
    The distance between the student and the mirror is now (5 – 1) = 4 m.
    Therefore, the distance between the student and his image is (4 m + 4 m) = 8 m.

What is Reflection of Light?

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What is Reflection of Light?

Light rays travel in a straight line. A ray is a very narrow beam of light.
An object can only be seen when light rays from the object enter our eyes. These light rays can come directly from the object or as a result of the object reflecting light rays from a light source.
What is Reflection of Light(a) The pictures from the television can be seen because the television emits light.
What is Reflection of Light 1(b) The picture can be seen because it reflects light from the light source.

Definition: When light rays are incident on an opaque  polished surface (medium), these are returned back in the same medium. This phenomenon of returning of ray of light in the same medium, is called reflection of light.
When you look at a mirror, you see the reflection of other things around you along with your own image.

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Definition of some associated terms:
Law of Reflection of light

  1. Reflecting surface: The surface from which the light is reflected, is called the reflecting surface. In diagram, XY is the reflecting surface.
  2. Point of incidence: The point on the reflecting surface at which a ray of light strikes, is called the point of incidence. In diagram, O is the point of incidence.
  3. Normal: A perpendicular drawn on the reflecting surface at the point of incidence, is called the normal. In diagram, ON is the normal.
  4. Incident ray: The ray of light which strikes the reflecting surface at the point of incidence is called the incident ray. In diagram, PO is the incident ray.
  5. Reflected ray: The ray of light reflected from the reflecting surface from the point of incidence, is called the reflected ray. In diagram, OQ is the reflected ray.
  6. Angle of incidence: The angle that the incident ray makes with the normal, is called the angle of incidence. It is represented by the symbol i. In diagram, angle PON is the angle of incidence.
  7. Angle of reflection: The angle that the reflected ray makes with the normal, is called the angle of reflection. It is represented by the symbol r. In diagram, ∠QON is the angle of reflection.
  8. Plane of incidence: The plane in which the normal and the incident ray lie, is called the plane of incidence. In diagram, the plane of the bookpage, is the plane of incidence.
    1. Plane of reflection: The plane in which the normal and the reflected ray lie, is called the plane of reflection. In diagram, the plane of the book page, is the plane of reflection.

Reflection of Light on a Plane Mirror

What is Reflection of Light 2

  1. When you look at the image of an object in a plane mirror, the rays of light originating from the object hit the mirror and bounce or reflect from the mirror towards your eye. These reflected rays produce the image that is seen in the mirror.
  2. The ray of light that strikes the mirror is known as the incident ray. The ray of light which reflects from the mirror is known as the reflected ray.
    What is Reflection of Light 3
  3. The normal is a line drawn perpendicularly (at a right-angle) to the surface of the reflector. The normal line divides the angle between the incident ray and the reflected ray into two equal angles.
  4. The angle between the incident ray and the normal is known as the angle of incidence (i). The angle between the reflected ray and the normal is known as the angle of reflection (r).
    Note: The angles of incidence and reflection are always measured from the normal.
  5. The behaviour of light when it is reflected follows a law known as the law of reflection.
    • The incident ray, the reflected ray and the normal to the surface lie in the same plane.
    • The angle of incidence is equal to the angle of reflection (∠i = ∠r).

Reflection of Light Experiment

Aim: To study the nature of reflection.
Materials needed: A4 size paper, a pair of scissors, adhesive tape, and a wall mirror (bathroom mirror, dressing mirror, etc.)
Method:

  1. Fold the A4 size paper in the middle, lengthwise.
  2. Cut out a thin slit on the fold, leaving out 1 inch on the top and bottom of the paper.
  3. Open out the paper and stick it on a mirror.
  4. Look at yourself in the mirror through the slit in the paper. Make a note of the various objects that you can see behind you.
    What is Reflection of Light?
  5. Now move to your right and look at the mirror through the slit. Try out various positions. Try to see along the surface of the mirror. Make a note of the images that you see at the various positions of your eye.

Observation: You will notice that you will be able to see yourself only if you are directly in front of the slit. If you move to the right, you will be able to see the image of things to your left. If you move further to the right, you will be able to see things further to your left.

Let us put down our observations from the above activity in a scientific manner, by drawing a diagram. First draw a line perpendicular (i.e., a line that makes an angle of 90°) to the mirror at the point where the slit is located. This line, SN, is called the normal.

The rays of light that come from the object and hit the mirror are called incident rays. In figure, AS and BS are incident rays. The rays of light that get reflected from (i.e., bounce off) the mirror are called reflected rays. In figure, SA’ and SB’ are reflected rays.
The point at which the incident ray hits the mirror is called the point of incidence. In figure, S is the point of incidence. A normal drawn to the mirror at the point of incidence is called a normal at the point of incidence. The angle between the incident ray and the normal is called the angle of incidence. In figure, angle ASN is the angle of incidence of the incident ray AS. The angle between the reflected ray and the normal is called the angle of reflection. In figure, the angle of reflection of the reflected ray SA’ is NS A’.

Refraction of Light Problems with Solutions

  1. Two plane mirrors are placed at right angles to each other as shown in Figure. A light ray is incident on one of the mirrors at 45°. Complete the path of the light ray in the figure. What can you say about the path of the incident ray and the final reflected ray?
    Refraction of Light Problems
    Solution:
    Refraction of Light Problems 1
    The incident ray and the final reflected ray are parallel but in opposite direction.
  2. Figure shows a light ray incident on mirror.
    Refraction of Light Problems 2
    What is the angle of reflection?
    Solution:
    Refraction of Light Problems 3Refraction of Light Problems 3
  3. A light ray is incident on a plane mirror with a 20° angle of incidence, as shown in Figure.
    Refraction of Light Problems 4
    If the mirror is rotated 10° clockwise, what is the angle turned by the reflected ray?
    Solution:
    Refraction of Light Problems 5
    The angle turned by the reflected ray = 20°
    Note: The angle turned by reflected ray is always ^twice the angle turned by the plane mirror.

Reflecting Surfaces

All types of surfaces reflect light. That is why we can see them. When light from the sun or any source falls on an object, we are able to see the object because the light reflected by the object reaches our eyes.
While talking of reflection, we refer to a smooth surface as a regular surface, and a rough and wavy surface as an irregular surface. A regular surface reflects light in only one direction. Reflection by a regular surface is referred to as regular reflection. A rough surface reflects a parallel beam of light incident upon it in all directions. The small bumps and irregularities on a rough surface cause each of the light rays to reflect in different directions. This kind of reflection is called irregular or diffused reflection.
Reflecting Surfaces

Reflecting Surfaces Activity

Aim: To observe reflection from different types of surfaces.
Materials needed: Glass, metal sheet, metal foil, white paper, and mirror.
Method:

  1. Take each object and stand in front of a sunlit window.
  2. Try to catch the rays of the sun on the object and project it onto a wall.
    What are the different kinds of images formed? Record your observations for each object.

 

 

Applications of Reflection of Light in Daily Life

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Applications of Reflection of Light in Daily Life

  1. Meters like ammeters and voltmeters use a mirror to avoid parallax error. The reading is taken from a position such that the image of the pointer is directly under the pointer.
    Applications of Reflection of Light in Daily Life
  2. The wing and rear-view mirrors of a car are made of a convex and a plane mirror respectively. The two wing mirrors enable the driver to see objects on both sides of the car. The rear-view mirror enables the driver to see things behind the car.
    Applications of Reflection of Light in Daily Life 1
  3. A microscope uses a mirror to reflect light to the specimen under the microscope.
    Applications of Reflection of Light in Daily Life 2
  4. An astronomical reflecting telescope uses a large parabolic mirror to gather dim light from distant stars. A plane mirror is used to reflect the image to the eyepiece.Applications of Reflection of Light in Daily Life 3
  5. Parabolic mirrors are used in torches and car headlamps as reflectors. A small lamp is placed at the focus point of the mirror to produce parallel rays.Applications of Reflection of Light in Daily Life 4
  6. Concave mirrors with long focal length can be used as shaving mirrors or make-up mirrors as they form magnified and upright images.Applications of Reflection of Light in Daily Life 5
  7. Concave mirrors are also used by dentists to examine the teeth of a patient. The concave mirror forms a magnified image of the teeth.
    Applications of Reflection of Light in Daily Life 6
  8. A convex mirror has a wider view than a plane mirror. Therefore, convex mirrors are used as driving mirrors and as shop security mirrors.
    Applications of Reflection of Light in Daily Life 8
  9. Convex mirrors are also used as blind corner mirrors on the road to help drivers view traffic around sharp corners.
    Applications of Reflection of Light in Daily Life 7
  10. An overhead projector as in figure uses a concave mirror to reflect light from the object to the screen.
    Applications of Reflection of Light in Daily Life 9

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Application of Reflection of Light in the Construction of Devices

Periscope

Applications of Reflection of Light in Daily Life 10

  1. A periscope is a device used to see objects over an obstacle. It is made up of two plane mirrors mounted in a long tube. Both mirrors are set parallel to each other at each corner of the tube and at an angle of 45° to the path of the light rays.
  2. Light from the object is reflected through 90° by each mirror before entering the eye of an observer.
  3. Periscopes are often used in double-decker buses to let the driver see the situation in the upper deck.
  4. Periscopes are also used in submarines to observe the surrounding areas above the water surface.

Kaleidoscope

  1. A kaleidoscope is an optical device that produces colourful patterns and designs.
  2. It consists of a tube of plane mirrors with one end closed. The tube can be in the form of a triangle, square, rectangle or hexagon as shown in Figure.
    Applications of Reflection of Light in Daily Life 11
  3. Pieces of coloured beads or pebbles are placed inside the tube. The observer looks in one end and light enters the other end, reflecting off the mirrors.
    Applications of Reflection of Light in Daily Life 12
  4. As the tube is rotated, the tumbling of the coloured beads presents the observer with varying colours and patterns. Any arbitrary pattern of objects shows up as a beautiful symmetrical pattern due to the reflections in the mirrors.
  5. The kaleidoscope is used by designers to yield carpet and wallpaper patterns.

An Illusion Box

  1. An illusion box is a device that can produce a virtual object in it.
  2. It consists of a concave mirror placed inside a box with its inner surfaces painted black as shown in Figure.
    Applications of Reflection of Light in Daily Life 13
  3. An optical illusion is produced when the box is viewed in a darkened room with the light source switched on.
  4. The object appears to hang in the space inside the box. When someone tries to touch the object, he or she will not be able to as it is actually not there.

Analysing Reflection of Waves

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Analysing Reflection of Waves

  1. Figure shows the reflection of sea waves by a retaining wall built near the beach.
    Analysing Reflection of Waves 1
  2. Reflection of waves is a phenomenon that occurs when all or a part of the waves return after they encounter an obstacle. The obstacle is known as a reflector.
  3. A ripple tank kit is a very useful apparatus for studying waves in the laboratory. Figure shows a ripple tank in use.
    Analysing Reflection of Waves 2(a) When a steady train of waves move across the water surface, each wave crest acts like a convex lens and concentrates the light on the screen as shown in Figure. Flence, a bright pattern on the screen corresponds to a crest of the waves.
    Analysing Reflection of Waves 3(b) A stroboscope is normally used to observe wave patterns on the screen. By rotating the stroboscope at the same frequency as the motor of the ripple tank kit, the motion of the waves can be frozen for more detailed observation.

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Characteristics of Reflected Waves

Figure shows incident waves being reflected by a reflector. The angle of incidence, i is the same as that of the angle of reflection, r.

  • Direction: The direction of the propagation of the waves changed after reflection.
  • Angle of reflection: The angle of reflection, r is equal to reflection the angle of incidence, i.
  • Wavelength: The wavelength of the reflected waves is the same as that of the incident waves.
  • Frequency: The frequency of the reflected waves is the same as that of the incident waves.
  • Speed: The speed of the reflected waves is the same as that of the incident waves.

Analysing Reflection of Waves 4

Reflection of Plane Waves Experiment

Aim: To study the characteristics of reflection of plane waves in a ripple tank, light and sound waves.
Material: White paper as screen
Apparatus: Ripple tank and its accessories, stroboscope, protractor
Method:

Analysing Reflection of Waves 5

  1. The ripple tank is arranged as shown in Figure.
  2. The legs of the ripple tank are adjusted so that the base of the tank is horizontal.
  3. A horizontal beam vibrator is lowered until it touches the surface of the water to create plane (straight) waves.
  4. The speed of the motor is adjusted to produce a train of waves that can be clearly observed on the screen with the help of a stroboscope.
  5. A straight reflecting barrier (reflector) is placed on the tank so that the angle of incidence of the waves, i = 30°.
    Analysing Reflection of Waves 6
  6. The angle of the reflected waves, r is measured and recorded.
  7. The wavelengths of the incident and reflected waves are observed and compared.
  8. Steps 5 to 7 are repeated for i = 45° and 60°.

Results:

Analysing Reflection of Waves 7

Observation:
The wavelength of the reflected waves is the same as the incident waves.
Discussion:

  1. From the results, the angle of reflection of the waves is equal to the angle of incidence of the waves.
  2. The wavelength of the waves does not change after reflection.
  3. The frequency of the incident and reflected waves are the same as that of the source of the waves.
  4. From 2 and 3, and by using the formula v = fλ, the speed of the reflected waves is the same as that of the incident waves.

Reflection of Light Experiment

Aim: To study the characteristics of reflection of light.
Materials: A piece of white paper, plasticine, pencil Apparatus: Plane mirror, ray box, power supply, protractor
Method:

Analysing Reflection of Waves 8

  1. The apparatus is set up as shown in Figure.
  2. The activity is carried out to determine the angle of reflection, r corresponding to the angle of incidence, i = 30°, 45° and 60°.

Results:Analysing Reflection of Waves 9Discussion:
From the results, the angle of reflection of the light, r is equal to the angle of incidence of the light, i.

Reflection of Sound Waves Experiment

Aim: To study the characteristics of reflection of sound waves.
Materials: Round cardboard tubes, a piece of soft board, a piece of plywood, a piece of white paper
Apparatus: Protractor, mechanical stopwatch as source of sound
Method:

Analysing Reflection of Waves 10

  1. The apparatus is set up as shown in Figure.
  2. A stopwatch is placed in a cardboard tube with one end open and the other end closed.
  3. The tube is arranged at an incident angle, i = 30°.
  4. Another cardboard tube with both ends open is adjusted so that one ear is placed close to one end of the tube as to hear the loudest possible ticking sound of the stopwatch.
  5. The angle of reflection, r of the sound wave is measured and recorded.
  6. Steps 3 to 5 are repeated for i = 45° and 60°.

Results:
Analysing Reflection of Waves 11Discussion:
From the results, the angle of reflection, r is equal to the angle of incidence, i of the sound waves.
Conclusions:

  1. The direction of propagation of the reflected waves is different from that of the incident waves.
  2. The angle of reflection, r is equal to the angle of incidence, i of the waves.