The phenomenon of interference and diffraction reveals that light has wave nature like sound. But wave may be classified into two classes: (l) Longitudinal wave (ii) Transverse wave. Sound
waves are longitudinal in nature. Now we shall investigate the nature of light waves.
In order to investigate the nature of light we first demonstrate which is explained below. Take a stretched string AB one end (A) of which is free while other end (B) is fixed. Let it passes through two slits S l and S2
Now set up a longitudinal wave by moving end A forward and backward along the string. If slit S2 is rotated in any direction then there is no charge in the amplitude of the wave.
Now, set up transverse wave by moving the end A perpendicular to the length of the string.
When S1 and S2 are parallel to each other, then there is no charge in the amplitude of wave coming out of slit. If slit S2 is rotated slowly either in clockwise or in anti-clockwise, then amplitude of the wave coming out of slit S2 diminishes. If S2 is perpendicular to S1 then there is no wave coming out of S2. Thus the transverse wave can pass through the slits when these slits are parallel to each other as shown in Figure 6.1.
Same type of behaviour is shown by the light when it passes through tourmaline crystal cut parallel to optic axis.
If we rotate the crystal Q either in clockwise or anti-clockwise, then intensity of light emerging out of tourmaline crystal Q decreases. When the axis of two crystals become perpendicular to each other then intensity of light emerging out of Q is zero. It is also noted that only vibrations parallel to crystallographic axis are allowed to pass through the crystal. Thus the vibrations are confined only in a particular direction.
Thus on the basis of above experiment one can say that light waves are transverse in nature i.e., the displacement associated with light waves is at right angle to the direction of propagation of the wave. The tourmaline crystal P is known as polariser while Q is known as analyzer.