Let us see an example

Example 2

A prism with an angle A=60° produces an angle of minimum deviation of 30°. Find the refractive index of material of the prism.

Solution:Given that A = 60° and D=30°.

Using n Sin[(A+D)/2]/Sin(A/2)= Sin(90°/2)/Sin(30)

=Sin 45% Sin30= (1/2)/(1/2)=√2

n=√2

thus, the refractive index of the given prism = √2

Let us take up a simple activity with prism.

Do this activity in the dark room. Take a prism and place it on the table near a vertical white wall. Take a thin wooden plank. Make a small hole in it and fix it vertically on the table. Place the prism between the wooden plank and wall. Place a white light source behind the hole of the wooden plank. Switch on the light. The rays coming out of the hole of plank become a narrow beam of light. Adjust the height of the prism such that the light falls on one of the lateral surfaces. Observe the changes in emerged rays of the prism. Adjust the prism by slightly rotating it till you get an image on the wall.

• What do you observe on the wall?

• Could you get a coloured image on the wall?

• Why does white light split into colours?

• What colours do you see?

Can you notice any change in the angle of deviation of each colour?

Which colour has the minimum deviation?

Let us do another activity

Take a metal tray and fill it with water. Place a mirror in the water such that it makes an angle to the water surface. Now focus white light on the mirror through the water as shown in figure 11. Try to obtain colours on a white card board sheet kept above the water surface. Note the names of the colours you could see in your book.