NDA (I & II) 2020 General Ability Test · Question 133 of 148

Correct — B, 'Sound waves are transverse waves' — this is the statement that is NOT correct. In air and other fluids, sound propagates as a longitudinal wave: the medium's particles oscillate back and forth along the direction the wave travels, creating compressions and rarefactions. Describing sound as transverse (particles vibrating perpendicular to travel, like light or a plucked string) is wrong, which is exactly the false statement the question asks you to find. Statements a, c and d are all true.

• (a)Sound travels at a speed slower than the speed of light — This is a true statement — sound in air is about 343 m/s, vastly slower than light's 3 × 10⁸ m/s — so it is not the incorrect statement being sought.
• (c)Sound waves are longitudinal waves — This is the correct description of sound; it is a true statement, so it cannot be the 'NOT correct' answer.
• (d)Sound travels faster in moist air than in dry air — This is true — humid air is slightly less dense than dry air because water vapour is lighter than nitrogen and oxygen, which raises the speed of sound; a true statement, not the odd one out.

Waves are classified by the direction of particle vibration relative to the wave's travel. In a longitudinal wave, particles vibrate parallel to the direction of propagation (compressions and rarefactions); in a transverse wave they vibrate perpendicular to it. Sound in fluids is longitudinal because gases and liquids can be compressed but cannot sustain the sideways shear that a transverse wave needs.

The 'NOT correct' framing pairs a true statement (c, longitudinal) with a false one (b, transverse) to test whether you know the true nature of sound. Because fluids have no shear rigidity, only compressional (longitudinal) waves travel through them, making 'transverse' the wrong label.

• Sound in air and other fluids is a longitudinal wave.
• Speed of sound in air ≈ 343 m/s, far slower than light (3 × 10⁸ m/s).
• Sound travels faster in moist air than dry air (water vapour lowers density).
• Transverse mechanical waves need shear rigidity, which fluids lack.

• The 'NOT correct' wording — you must pick the false statement, and both 'transverse' and 'longitudinal' are offered as bait.
• Assuming dry air carries sound faster than moist air — the reverse is true.

Asked as an odd-one-out on the properties of sound — spot that 'transverse' is the wrong description for sound in a fluid.

• practice — not a real PYQ

  Sound cannot travel through

  • (a)a solid
  • (b)a liquid
  • (c)a gas
  • (d)a vacuum

  Answer(d) a vacuum — sound is a mechanical wave and needs a material medium to travel.

• practice — not a real PYQ

  In which medium does sound generally travel fastest?

  • (a)Steel (solid)
  • (b)Water (liquid)
  • (c)Air (gas)
  • (d)Vacuum

  Answer(a) Steel (solid) — sound travels fastest in solids because of their high elasticity.