Transverse Wave Polarization
The experiments on interferences and diffraction have shown that light is a form of wave motion. These effects do not tell us about the type of wave motion i.e. whether the light waves are longitudinal or transverse, or whether the vibrations are linear, circular or torsional. The phenomenon of polarization has helped to establish beyond doubt that light waves are transverse waves.
Let a rope AB be passed through two parallel slits S1 and S2. The rope is attached to a fixed point at B. hold the end A and move the rope up and down perpendicular to AB. A wave emerges along CD and it is due to transverse vibrations parallel to the slit S1. The slit S2 allows the wave to pass through it when it is parallel to S1. It is observed that the slit S2 does not allow the wave to pass through it when it is at right angles to the slit S1.
If the end A is moved in a circular manner, the rope will show circular motion up to the slit S1. Beyond S1, it will show only linear vibrations parallel to the slit S1, because the slit S1 will stop the other components. If S1 and S2 are at right angles to each other the rope will not show any vibration beyond S2.
If longitudinal waves are set up by moving the rope forward and backward along the string, the waves will pass through S1 and S2 irrespective of their positions.
A similar phenomenon has been observed in light when it passes through a tourmaline crystal.
Let light from a source S fall on a tourmaline crystal A which is cut parallel to its axis. The crystal A will act as the slit S1. The light is slightly coloured due to the natural colour of the crystal. On rotating the crystal A, no remarkable change is noticed. Now place the crystal B parallel to A.
1. Rotate both the crystals together so that their axes are always parallel. No change is observed in the light coming out of B.
2. Keep the crystal A fixed and rotate the crystal B. The light transmitted through B becomes dimmer and dimmer. When B is at right angles to A, no light emerges out of B.
If the crystal B is further rotated, the intensity of light coming out of it gradually increases and is maximum again when the two crystals are parallel.
This experiment shows conclusively that light is not propagated as longitudinal or compressional waves. If we consider the propagation of light as a longitudinal wave motion then no extinction of light should occur when the crystal B is rotated.
It is clear that after passing through the crystal A, the light waves vibrate only in one direction. Therefore light coming out of the crystal A is said to be polarized because it has acquired the property of one-sidedness with regard to the direction of the rays.
This experiment proves that light waves are transverse waves, otherwise light coming out of B could never be extinguished by simply rotating the crystal B.
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