# How will you use two identical prisms

Question:

How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the sec¬ond prism as white light ? Draw the diagram.

Solution:

Perform an activity to show that the colours of white light splitted by a glass prism can be recombined to get white light by another glass prism.
Apparatus required. Two glass prisms made of same kind of glass, a card board having a fine hole at its centre, a white screen.
Procedure:

1. Place a card board in front of a prism A. A ray of white light coming from the hole in the card board falls on the prism A (Figure 11).

2. White light splits into seven colours by prism A is made to fall on another glass prism B placed with its base upward. Since prism A disperses white light, so it is known as dispersing prism.

3. The prism B deviates colours of light towards its base. The various colours recombines at the opposite lace of glass prism B.

4. This activity was initially performed by the great scientist Issac Newton.

The light received on the white screen placed in front of prism B is white. Since the prism B recombines the colours of light to produce white light, so glass prism $B$ is known as recombination prism.