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# Multiple Reflections

The number of images formed, depends on the angle of inclination of the mirrors. If the angle between the two mirrors is a factor of 360°, then the total number of reflections is finite.

Multiple Reflections

Activity 4

Take two plane mirrors and keep them perpendicular to each other. Place an object between them. You can see the images of the object. How many images do you see in the mirrors? You can see three images. How is it possible to have three images with two mirrors?

Aim : To understand the concept multiple reflection.

Materials required : Two plane mirrors, an object.

Procedure :

(i) Take two plane mirrors and keep them perpendicular to each other.

(ii) Place an object between them.

Observation : We can see three images of the object.

Inference :

(i) When a body kept in between two plane mirrors, which were inclined to each other. You could see many images. This is because, the image formed by one mirror acts as an object for the other mirror.

(ii) The image formed by the first mirror acts as an object for the second mirror and the image formed by the second mirror acts as an object for the first mirror.

(iii) Thus we have three images of a single body. This is known as multiple reflection.

In the activity given above, you observed that for an object kept in between two plane mirrors, which were inclined to each other, you could see many images. This is because, the ‘image’ formed by one mirror acts as an ‘object’ for the other mirror. The image formed by the first mirror acts as an object for the second mirror and the image formed by the second mirror acts as an object for the first mirror. Thus, we have three images of a single body. This is known as multiple reflection. This type of reflections can be seen in show rooms and saloons.

The number of images formed, depends on the angle of inclination of the mirrors. If the angle between the two mirrors is a factor of 360°, then the total number of reflections is finite. If θ (Theta) is the angle of inclination of the plane mirrors, the number of images formed is equal to [360°/θ] – 1 . As you decrease this angle, the number of images formed increases. When they are parallel to each other, the number of images formed becomes infinite.

Problem 3

If two plane mirrors are inclined to each other at an angle of 90°, find the number of images formed.

Solution

Angle of inclination = 90°

Number of umber of images formed = [360°/θ] – 1

= [360°90°] –1= 4-1=3

1. Kaleidoscope

It is a device which functions on the principle of multiple reflection of light, to produce numerous patterns of images. It has two or more mirrors inclined to each other. It can be designed from inexpensive materials. The colourful image patterns formed by this will be pleasing to you. This instrument is used as a toy for children.

Activity 5

Take three equal sized plane mirror strips and arrange them in such a way that they form an equilateral triangle. Cover the sides of the mirrors with a chart paper. In the same manner cover the bottom of the mirrors also. Put some coloured things such as pieces of bangles and beads inside it. Now, cover the top portion with the chart paper and make a hole in it to see. You can wrap the entire piece with coloured papers to make it attractive. Now, rotate it and see through its opening. You can see the beautiful patterns.

Caution: Be careful while handling the glass pieces. Do this under the supervision of your teacher.

Aim : To construct kaleidoscope and to produce numerous patterns of images.

Materials required : Three equal sized plane mirrors, chart paper, pieces of bangles and beads, coloured papers.

Procedure :

(i) Take three equal sized plane mirror strips and arrange them in such a way that they form an equilateral triangle.

(ii) Cover the sides of the mirrors with a chart paper.

(iii) With the help of a chart paper cover the bottom of the mirrors also.

(iv) Put some coloured things such as pieces of bangles and beads inside it.

(v) Now, cover the top portion with the chart paper and make a hole in it to see.

(vi) You can wrap the entire piece with coloured papers to make it attractive.

(vii) Now, rotate it and see through its opening. You can see the beautiful patterns.

Observation : We can see the beautiful image patterns.

Inference : Kaleidoscope is a device, which functions on the principle of multiple reflection of light to produce numerous patterns of images.

2. Periscope

It is an instrument used for viewing bodies or ships, which are over and around another body or a submarine. It is based on the principle of the law of reflection of light. It consists of a long outer case and inside this case mirrors or prisms are kept at each end, inclined at an angle of 45°. Light coming from the distant body, falls on the mirror at the top end of the periscope and gets reflected vertically downward. This light is reflected again by the second mirror kept at the bottom, so as to travel horizontally and reach the eye of the observer. In some complex periscopes, optic fibre is used instead of mirrors for obtaining a higher resolution. The distance between the mirrors varies depending on the purpose.

Uses

* It is used in warfare and navigation of the submarine.

* In military it is used for pointing and firing guns from a ‘bunker’.

* Photographs of important places can be taken through periscopes without trespassing restricted military regions.

* Fibre optic periscopes are used by doctors as endoscopes to view internal organs of the body.

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