CDS (I) 2023 General Knowledge · Question 47 of 120

Correct — A, respectively longitudinal and transverse in air. Sound is a mechanical wave: it needs matter to travel through, and it moves by compressing and rarefying that matter along the direction it travels. The particles of air oscillate back and forth along the line of travel, not across it, which is the definition of a longitudinal wave. There is a physical reason air leaves no choice in the matter — a transverse wave requires the medium to resist shearing, and a gas cannot. Light is a different kind of wave altogether. It is an electromagnetic wave, an oscillating electric field with a magnetic field at right angles to it, and both fields point across the direction of travel. That makes light transverse, and it needs no medium at all, which is how sunlight crosses the vacuum between the Sun and the Earth. The decisive experimental evidence that light is transverse is polarisation: light can be filtered so that its oscillation is confined to one plane, and a purely longitudinal wave cannot be polarised in that way. Sound cannot be polarised, and that single contrast settles the pair.

• (b)respectively transverse and longitudinal in air — The right two descriptions in the wrong order, which is the error the option exists to catch. Sound in air is longitudinal and light is transverse, not the reverse.
• (c)both longitudinal in air — Correct for sound, wrong for light. If light were longitudinal it could not be polarised, and polarising filters plainly work.
• (d)both transverse in air — Correct for light, wrong for sound. A gas cannot sustain a shear stress, so it cannot carry a transverse mechanical wave at all.

In a longitudinal wave the particles of the medium oscillate along the direction in which the wave travels, producing alternating compressions and rarefactions; sound in air is the standard example. In a transverse wave the oscillation is perpendicular to the direction of travel; light and all electromagnetic waves are transverse, as are waves on a stretched string. Mechanical waves need a medium; electromagnetic waves do not.

The trap here is the word 'respectively', and the fix is to settle each half separately before matching them to the order. The useful physical rule behind the answer is about the medium rather than about the wave: fluids — gases and liquids — cannot resist shear, so a mechanical wave passing through them can only be longitudinal, while a solid can carry both. That is why an earthquake sends both a longitudinal primary wave and a transverse secondary wave through rock, but only the primary wave crosses the Earth's liquid outer core. UPSC has tested precisely that point, offering the claim that sound travels in rock as longitudinal waves only and expecting it to be rejected.

• Sound is a mechanical wave and is longitudinal in air, travelling as compressions and rarefactions.
• Light is an electromagnetic wave and is transverse, with electric and magnetic fields oscillating at right angles to the direction of travel and to each other.
• Light needs no medium and travels through a vacuum; sound cannot travel through a vacuum.
• Polarisation is the phenomenon that demonstrates light is transverse; longitudinal waves cannot be polarised.
• Gases and liquids cannot sustain shear, so mechanical waves in them are longitudinal only, while solids can carry both longitudinal and transverse waves.

• Reversing the pair in a respectively question.
• Assuming a transverse mechanical wave can travel through air; a gas cannot sustain shear.
• Treating diffraction or interference as proof that light is transverse — every wave shows those; only polarisation settles it.

As a respectively pairing on sound and light, as a which-phenomenon-proves-light-transverse item, or as a not-correct statement on the nature of sound.

• practice — not a real PYQ

  Which one of the following phenomena demonstrates that light is a transverse wave?

  • (a)Reflection
  • (b)Refraction
  • (c)Interference
  • (d)Polarisation

  Answer(d) Polarisation — reflection, refraction and interference are shown by longitudinal waves too, but only a transverse wave can be confined to a single plane of oscillation.

• practice — not a real PYQ

  Sound cannot travel through a vacuum because

  • (a)a vacuum absorbs sound energy
  • (b)sound is a mechanical wave and needs a material medium
  • (c)the speed of sound in a vacuum is infinite
  • (d)sound waves are transverse

  Answer(b) sound is a mechanical wave and needs a material medium — there are no particles in a vacuum to be compressed and rarefied.