Physics · Wave Optics · NEET
In a transverse wave the vibration is at right angles to the travel direction, so there is a real choice of sideways direction (up-down, left-right, or any angle). A polaroid can pass only one of these directions and block the rest, so rotating it changes the output. In a longitudinal wave the vibration is along the travel direction only, so there is nothing sideways to select or block. Because polaroids clearly do select a direction in light, the vibration must be sideways, meaning light is transverse.
Send unpolarised light through the first polaroid P2. It comes out linearly polarised and its intensity drops to half. Now put a second polaroid P1 after it and slowly rotate P1. When the pass axes are parallel you get maximum light. When you rotate P1 to 90 degrees (crossed), the transmitted intensity becomes nearly zero, so the field goes dark. This dark-when-crossed result is only possible if light vibrates sideways in a definite direction, which is the signature of a transverse wave.
Sound in air is a longitudinal wave. The air layers vibrate back and forth along the same line the sound travels, so the vibration has no sideways direction to pick out. A polariser works by selecting one sideways direction, but a longitudinal wave has none. So no filter can ever polarise sound. The fact that light CAN be polarised is exactly what separates it from sound and proves light is transverse.
Be careful here. Interference and diffraction prove that light is a wave. Polarisation goes one step further and proves that this wave is transverse (vibration perpendicular to travel), because only transverse waves can be polarised. So for NEET: wave nature is shown by interference and diffraction, transverse nature is shown by polarisation.
A polaroid is made of long chain molecules aligned in one direction. The electric field of light that vibrates ALONG the aligned molecules gets absorbed. The field vibrating perpendicular to the molecules passes through. That perpendicular direction is called the pass axis. So a polaroid only lets through the electric vector along its pass axis; the component along the molecule chains is absorbed.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It establishes the transverse nature of light, meaning the electric field vibrates at right angles to the direction the light travels.
No. Longitudinal waves vibrate along the travel direction, so there is no sideways direction to filter. Only transverse waves can be polarised.
When two polaroids have their pass axes at 90 degrees, the transmitted light intensity is nearly zero, so the view goes dark.
An unpolarised beam passing through one ideal polaroid comes out with half its intensity, and rotating that single polaroid does not change the output.
Yes. All electromagnetic waves, including light, radio, and X-rays, are transverse and can be polarised.
