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You might have seen a beam of sunlight when it enters a room through a narrow opening or a hole. You may have also seen beams of light from the headlamps of scooters, cars and engines of trains [Fig. 15.1 (a)]. Similarly, a beam of light can be seen from a torch. Some of you may have seen a beam of searchlight from a light house or from an airport tower [Fig. 15.1 (b)].
What do these experiences suggest?
15.1 LIGHT TRAVELS ALONG A STRAIGHT LINE
Boojho recalls an activity he performed in Class VI. In that activity he looked at a lighted candle first through a straight pipe and then through a bent pipe (Fig. 15.2). Why was Boojho not able to see the candle flame through a bent pipe? This activity showed that light travels along straight lines. How can we change the path of light? Do you know, what happens when light falls on a polished or a shiny surface?
15.2 REFLECTION OF LIGHT
One way to change the direction of light is to let it fall on a shiny surface. For example, a shining stainless steel plate or a shining steel spoon can change the direction of light. The surface of water can also act like a mirror and change the path of light. Have you ever seen the reflection of trees or buildings in water Any polished or a shiny surface can act as a mirror. What happens when light falls on a mirror? You have learnt in Class VI that a mirror changes the direction of light that falls on it. This change of direction by a mirror is called reflection of light. Can you recall the activity in which you got the light of a torch reflected from a mirror? Let us perform a similar activity.
1. Fill in the blanks:
(a) An image that cannot be obtained on a screen is called ____________.
(b) Image formed by a convex __________ is always virtual and smaller in size.
(c) An image formed by a __________ mirror is always of the same size as that of the object.
(d) An image which can be obtained on a screen is called a _________ image.
(e) An image formed by a concave ___________ cannot be obtained on a screen.
2. Mark ‘T’ if the statement is true and ‘F’ if it is false:
(a) We can obtain an enlarged and erect image by a convex mirror. (T/F)
(b) A concave lens always form a virtual image. (T/F)
(c) We can obtain a real, enlarged and inverted image by a concave mirror. (T/F)
(d) A real image cannot be obtained on a screen. (T/F)
(e) A concave mirror always form a real image. (T/F)
3. State the characteristics of the image formed by a plane mirror.
4. Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.
5. What is a virtual image? Give one situation where a virtual image is formed.
6. State two differences between a convex and a concave lens.
7. Give one use each of a concave and a convex mirror.
8. Which type of mirror can form a real image?
9. Which type of lens forms always a virtual image?
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