Objective

This topic gives an overview of; 

• Light
• Light Travels along a Straight Line
• Reflection of Light

Light

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. 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.What do these experiences suggest?

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. 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?

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.

Activity 1

Take a torch. Cover its glass with a chart paper which has three slits . Spread a sheet of chart paper on a smooth wooden board. Fix a plane mirror strip vertically on the chart paper ). Now direct the beam of light on the mirror from the torch with slits. Place the torch in such a way that its light is seen along the chart paper on the board. Now adjust its position so that the light from the torch strikes the plane mirror at an angle .

Does the mirror change the direction of light that falls on it? Now move the torch slightly to either side. Do you find any change in the direction of reflected light?

Look into the mirror along the direction of the reflected light. Do you see the slits in the mirror? This is the image of the slits.

This activity shows how light gets reflected from a plane mirror.

Let us play around with the images formed in mirrors and know a little more about them.

Activity 2

Place a lighted candle in front of a plane mirror. Try to see the flame of the candle in the mirror. It appears as if a similar candle is placed behind the mirror. The candle, which appears behind the mirror, is the image of the candle formed by the mirror. The candle itself is the object.

Now move the candle to different positions in front of the mirror. Observe the image in each case.

Was the image upright in each case? Did the flame appear on top of the candle as in the object? Such an image is called erect. An image formed by a plane mirror is erect and of the same size as the object.

Now place a vertical screen behind the mirror. Try to obtain the image of the candle on this screen. Can you get the image on the screen? Now place the screen in front of the mirror. Can you get the image on the screen now? You will find that the image of the candle cannot be obtained on the screen in either case.

What about the distance of the image from mirror? Let us perform another activity.

Activity 3

Take a chess board. If a chess board is not available, draw on a chart paper 64 (8◊8) squares of equal size. Draw a thick line in the middle of the paper. Fix a plane mirror vertically on this line. Place any small object, such as a pencil sharpner, at the boundary of the third square counting from the mirror. Note the position of the image. Now shift the object to the boundary of the fourth square. Again note the position of the image. Did you find any relation between the distance of the image from the mirror and that of the object in front of it?

Right Or Left !

When you see your image in a plane mirror, is it exactly like you? Have you ever noticed that there is one interesting difference between you and your image in a mirror? Let us find out.

Activity 

Stand in front of a plane mirror and look at your image. Raise your left hand. Which hand does your image raise ? Now touch your right ear. Which ear does your hand touch in your image? Observe carefully. You will find that in the mirror the right appears left and the left appears right. Note that only sides are interchanged; the image does not appear upside down.Now write down your name on a piece of paper and hold it in front of a plane

Can you now understand why the word AMBULANCE is written as in the picture? When the driver of a vehicle ahead of an ambulance looks in her/his rear view mirror, she/he can read AMBULANCE written on it and give way to it. It is the duty of every one of us to allow an ambulance to pass without blocking its way.

You might have observed that in the side mirror of a scooter or a car the images of all the objects appear smaller than the objects themselves. Have you ever wondered why it is so?

Summary

• Light travels along straight lines.
• Any polished or a shining surface acts as a mirror.
• An image which can be obtained on a screen is called a real image.
• An image which cannot be obtained on a screen is called a virtual image.