Chapter 11: Light, Shadows and Reflections

    I. Multiple Choice Questions (MCQs).
    Q.1. The objects that can emit light of their own is called
    a) lightning
    b) colourful
    c) luminous
    d) non-luminous
    Answer: (c) luminous
    Q.2. The sources of light such as sun, stars, glow worms, etc., are
    a) natural sources
    b) artificial sources
    c) non-luminous
    d) transparent

    Answer – a) natural sources
    Q.3. The path along which the light energy travels in a given direction
    a) beam of light
    b) ray of light
    c) both (a) and (b)
    d) none of these
    Answer: b) ray of light
    Q.4. The region of darkness formed when an opaque object blocks light is called
    a) shadow
    b) image
    c) point source
    d) all of these
    Answer: a) shadow
    Q.5. Which of these objects can cast a shadow?
    a) A transparent plastic sheet
    b) A glass pane
    c) A metal block
    d) Pure water
    Answer: c) A metal block

    II. Fill in the blanks by selecting appropriate words from the given list :
    List : Pinhole, black, grey, 343, $3 × 10^{8}$, shadows
    1. Velocity of light in air is _______________ m/s.
    2. Velocity of sound in air is _______________ m/s.
    3. The colour of shadows cast by the objects are always _________ or _________.
    4. Light, obstacle and screen are necessary for the formation of ___________.
    5. _______________ camera is the simplest camera without lens.

    Ans. 1- $3 × 10^{8}$,
    2-343,
    3-black, grey
    4- shadows,
    5- Pinhole

    III. Match the items of Column A with those of Column B.

    Answer: 1-c, 2-d, 3-b, 4-e, 5-a

    IV. Write ‘T’ for the true and ‘F’ for the false statements.
    Q.1. Non-luminous objects are those which cannot emit light of their own.
    Q.2. Materials through which things can be partially seen are called transparent materials.
    Q.3. A collection of large number of rays of light is called a beam of light.
    Q.4. Light travels in a straight line.
    Q.5. Shadow of an object does not depend on the distance between source of light and obstacle.

    Answer: 1-T, 2-F, 3-T, 4-T, 5-F
    V. Very Short Answer Questions.

    Q.1. Write two natural sources of light.
    Answer: Sun and Stars.

    Q.2. Write two artificial sources of light.
    Answer: Candle and Bulb

    Q.3. How can non-luminous objects be made luminous?
    Answer: Any non-luminous object can be made luminous by heating it.

    Q.4. Write two examples of transparent materials.
    Answer: Glass and Water

    Q.5. Write two examples of translucent materials.
    Answer: Oiled paper and tissue paper.

    VI. Short Answer Questions.

    Q.1. What do you mean by ray and beam?
    Answer: The path along which the light energy travels in a given direction is called a ray of light. It is shown by a straight line.

    A collection of large number of rays of light is called a beam of light.

    Q.2. Describe with an experiment that light travels in a straight line.
    Answer: To show that light travels in a straight line we need a long rubber tube and electric bulb. Now switch on the electric bulb. Now look at a bulb through the rubber tube. You could see the lighted bulb clearly. Bend the tube a little while you are looking at the bulb. We cannot see the bulb. On turning the pipe a little to your right or left, we cannot see the bulb. This is because light travels along a straight line and it cannot bend around.

    Q.3. What is a shadow? What are necessary conditions for the formation of a shadow?
    Answer: When an opaque object is placed on the path of light, region of darkness or partially light is caused. This region of darkness or partially light is called shadow.
    Necessary conditions for the formation of a shadow

    a) Source of light
    b) Opaque object and
    c) Screen
    Q.4. What do you mean by optical medium? Explain its kinds with proper examples.
    Answer: A material through which light can pass is called an optical medium.
    On the basis of their behaviour towards light, different media can be classified into three categories:

    i. Transparent substances: A substance through which light can be easily transmitted, making the objects to be seen clearly is called transparent substance. For example, air, water, glass, etc.

    ii. Opaque substances: A substance which does not allow light to pass through it is called an opaque substance. For example, wood, metal, stone, etc.

    iii. Translucent substances: A substance through which light passes only partially and objects are not clearly seen is called a translucent substance. For example, wax paper, frosted glass, clouds etc

    Q.5. Describe kinds of beam of light with suitable examples.
    Answer: There are three kinds of beam of light:
    (a) Parallel beam of light,
    (b) Convergent beam of light, and
    (c) Divergent beam of light.
    (a) Parallel beam of light: When a large number of rays are parallel to each other, they are called a parallel beam of light.

    (b) When rays of light coming from different directions, meet at a point, such a collection of rays is called a convergent beam of light.

    (c) When the rays of light starting from a point travel in various directions, the collection of such rays is called a divergent beam of light.

    Q.6. Write differences between image and shadow.

    Answer: Differences between the Image and Shadow

    VII. Long Answer Questions.

    Q.1. Write the factors on which the shape and size of a shadow depend. Describe with an experiment that how to observe the shape and size of shadows.

    Answer: Shadow of an object depends on the distance between source of light and obstacle or between obstacle and screen, and on the angle between the source of light and the obstacle. This can be proved by the following activity.

    To observe the shape and size of shadows you need a source of light, long stick opaque body like a shape of bird.

    Procedure : Take a long stick and look at its shadow on the ground. You can also observe the shadow by moving the stick on the ground. You will notice that the size of the shadow changes. Now point the stick towards the sun. You will notice that the shadow is diminished to a point. It becomes longer as you increase the angle between the rays

    of the sun and the stick.

    Take a bird shape opaque body. Shine the torch and hold the body in front of it. A shadow appears. Now move the torch nearer the body. The shadow of the bird grows.

    Q.2. (a) Discuss how to make a pinhole camera and the nature of image formation by it.
    (b) How can you see solar eclipse by pinhole camera?

    Answer: a) To make a pinhole camera. We need two rectangular cardboard boxes, a pair of scissors, a sharp-edged knife, glue, candle and black cloth.

    Procedure: Take two cardboard boxes similar in shape but different in sizes such that one can slide into another with no gap in between them. Cut out one side of each box. On the opposite face of the large box, make a small hole in the middle. In the opposite face of smaller box, cut out from the middle a square and fix a tracing paper over it with glue. Now slide the smaller box inside the larger box by keeping tracing paper inside. Now paint smaller box with black colour from inside. Dry it in sunlight. When the paint dry, your pinhole camera is ready for use.
    Now place a candle on a table and light it. Place the pinhole camera, a few inches away from the candle, with the side having the hole facing the candle.
    Look through the open face of the smaller box by covering your head and open face with black cloth.
    You will see an inverted and colourful image of the candle on the tracing paper screen. Move the smaller box forward to get a sharp image of candle. Now move the camera towards a distance building or tree. Your will observe that image is colourful, inverted and very small in size.

    b) Adjust your pinhole camera at a suitable place before the eclipse is to occur. When eclipse begins, look at the screen for image through the smaller box by covering it and head with black cloth. You will notice that apart of sun’s image gradually becomes dark and then disappears. After some time, it again appears and gradually grows in size and finally becomes a circular image of the sun.

    Q.3. (a) What do you mean by reflection and its kinds?
    (b) Describe with an experiment to show the reflection of light.

    Answer: (a) The bouncing off light from a surface is called reflection.
    There are two kinds of reflection
    Regular reflection : A smooth surface reflects a parallel beam of light in a particular direction. This type of reflection is called regular reflection.

    Diffused reflection: A rough surface reflects a parallel beam of light in all directions.
    This type of reflection is called diffused or irregular reflection.

    b) To show the reflection of light we need a plane mirror, polished brass plate, a book and torch.
    Go into a dark room with two friends. Now ask one of your friend to hold plane mirror and stand away from you at one corner of the room. Now ask the other friend to stand at another corner of room. Now switch on the torch and fall light on the mirror holding by your friend. What happens? You see that the beam of light reflects from the mirror. Now ask your friend holding plane mirror to rotate the mirror gently so that a bright patch of light is formed on your second friend.
    Now ask to rotate mirror in various directions. You will notice that reflected light also rotates.

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