NDA (II) 2023 General Ability Test · Question 134 of 146

Correct — D, Polarization. Polarization is a property of transverse waves, and it specifies the geometrical orientation of the oscillations. In a transverse wave the oscillation is perpendicular to the direction of travel, so there is a plane it can be confined to, and a filter can pass one orientation while blocking another. A longitudinal wave has no such freedom — its displacement is always along the direction of travel, so there is nothing to orient and no polarization to observe. Sound in a gas or liquid therefore cannot be polarized at all. Because light can be polarized, light must be oscillating across its line of travel, and that is the whole argument. The other three phenomena occur for every kind of wave and so decide nothing.

• (a)Refraction — Refraction is the bending of a wave when its speed changes at a boundary, and it happens to sound as readily as to light. It establishes that light is a wave with a finite speed, but says nothing about the direction in which it oscillates.
• (b)Diffraction — Diffraction is the spreading of a wave past an obstacle or through an aperture. Sound diffracts too, which is why you can hear round a corner without seeing round it. It proves wave behaviour, not transverse wave behaviour.
• (c)Interference — Interference is the superposition of two coherent waves into bright and dark fringes, and again it is common to longitudinal and transverse waves alike; the beats and dead spots you hear from two loudspeakers are sound interference. Young's experiment settled that light is a wave, but it left the transverse question open.

Reflection, refraction, diffraction and interference are the shared property of all waves, so any of them will tell you that light is a wave and none of them will tell you what kind. Polarization is the one phenomenon that separates the two families, because only a transverse wave has an oscillation direction that can be picked out and filtered. Light is an electromagnetic wave, consisting of a coupled oscillating electric field and magnetic field that are always perpendicular to each other, and both are perpendicular to the direction of travel — which is precisely why light can be polarized.

The history is the best way to hold this. Young's double-slit interference and the diffraction work of the early nineteenth century convinced physicists that light is a wave, but everyone at first pictured it as a longitudinal wave in some medium, by analogy with sound. What broke that picture was polarization — the observation that light passed through certain crystals or reflected at particular angles behaves differently depending on its orientation. A longitudinal wave cannot behave differently depending on orientation, because it has none. So in the exam, when a question asks which phenomenon supports the transverse nature specifically, ignore every option that sound also shows and pick the one it cannot.

• Polarization is a property of transverse waves which specifies the geometrical orientation of the oscillations.
• In longitudinal waves such as sound in a liquid or gas, the displacement is always along the direction of propagation, so these waves do not exhibit polarization.
• Light is an electromagnetic wave consisting of a coupled oscillating electric field and magnetic field that are always perpendicular to each other.
• Reflection, refraction, diffraction and interference are shown by longitudinal waves as well, so none of them can establish that light is transverse.

• Choosing interference or diffraction because they are the classic proofs that light is a wave; they do not decide whether it is transverse.
• Assuming sound cannot diffract or interfere, when it does both.
• Forgetting that a longitudinal wave has no orientation to filter, which is the whole reason polarization is decisive.

Asked as a single-concept discrimination item — pick the one wave phenomenon that only a transverse wave can show.

• practice — not a real PYQ

  Which one of the following waves cannot be polarized?

  • (a)Radio waves
  • (b)X-rays
  • (c)Sound waves in air
  • (d)Visible light

  Answer(c) Sound waves in air — being longitudinal, they have no oscillation across the line of travel to orient.

• practice — not a real PYQ

  Polarizing sunglasses cut glare from a wet road mainly because

  • (a)they absorb all wavelengths equally
  • (b)light reflected off a surface is partly polarized and the filter blocks that orientation
  • (c)they refract the incoming light away from the eye
  • (d)they diffract the reflected light into a wider beam

  Answer(b) light reflected off a surface is partly polarized and the filter blocks that orientation — the reason glare drops while the rest of the scene stays visible.