Question

Name the phenomenon that proves the transverse nature of light.

• A. Diffraction
• B. Interference
• C. Polarisation
• D. Reflection

Hint:

Think about which everyday wave behaviours need the wave to oscillate in one definite plane perpendicular to its direction of travel. A behaviour that can fully block a wave depending on a filter orientation works only for waves that move side to side, not for waves like sound that oscillate along their direction of travel.

Answer 1

The Correct Option is C

Concept: Light exhibits several wave phenomena such as reflection, refraction, interference, diffraction, and polarisation. Among these, polarisation is the phenomenon that specifically proves that light waves are transverse in nature. In transverse waves, the particles of the medium vibrate perpendicular to the direction of propagation. Only transverse waves can be polarised because their vibrations occur in multiple possible directions perpendicular to the wave's direction of travel.

Step 1: Understanding polarisation. Polarisation is the phenomenon in which the vibrations of light waves are restricted to a single plane perpendicular to the direction of propagation. Normally, light waves vibrate in many different planes. When light passes through a polarising material, only vibrations in one particular plane are allowed to pass through.

Step 2: Relation to transverse waves. This restriction of vibrations is possible only if the wave has oscillations perpendicular to its direction of motion. Therefore, the existence of polarisation confirms that light behaves as a transverse wave. Longitudinal waves such as sound cannot exhibit polarisation because their oscillations occur parallel to the direction of propagation.

Step 3: Conclusion. Since only transverse waves can be polarised, the observation of polarisation in light proves that light is transverse in nature. \[ \boxed{\text{Polarisation}} \]

Answer 2

Concept:

• A simple mechanical test separates transverse waves from longitudinal waves: pass the wave through a narrow slit and rotate the slit. A transverse wave, whose oscillation has a definite plane, is blocked when the slit is turned perpendicular to that plane. A longitudinal wave, whose oscillation runs along the direction of travel, passes through no matter how the slit is turned.
• Malus Law, $I = I_0\cos^2\theta$, gives a quantitative version of this same test for light: it shows the transmitted intensity depends on the angle between two polaroids, which is only possible if light has a definite oscillation plane.

Step 1: Apply the slit test to a transverse wave.
A rope wave shaken up and down passes freely through a vertical slit but is blocked by a horizontal slit, because its oscillation occupies one definite plane that can be aligned with or against the slit.

Step 2: Apply the same slit test to a longitudinal wave.
A longitudinal wave such as sound, produced by compressions and rarefactions along the direction of travel, passes through a slit of any orientation, since it has no side to side component that a slit could block.

Step 3: Connect this test to light using Malus Law.
When light passes through one polaroid, only the component oscillating along its transmission axis gets through. Passing this light through a second polaroid set at angle $\theta$ to the first gives transmitted intensity $I = I_0\cos^2\theta$, which falls to zero at $\theta = 90^\circ$. This angle dependent blocking, observed experimentally for light, is only possible if light waves oscillate in a definite plane transverse to their direction of travel.

Final Answer: Polarisation