Which among the following is true for propagation of sound waves?
- (a)Sound can travel in vacuum and it is a transverse wave in air.
- (b)Sound cannot travel in vacuum and it is a longitudinal wave in air.
- (c)Sound can travel in vacuum and it is a longitudinal wave in air.
- (d)Sound cannot travel in vacuum and it is a transverse wave in air.
Correct — B, sound cannot travel in vacuum and it is a longitudinal wave in air. Sound is a mechanical wave: it propagates only when particles of a material medium vibrate and pass the energy on to their neighbours. A vacuum has no particles, so sound cannot cross it. In air the particles oscillate back and forth along the same line the wave travels, creating compressions and rarefactions — that is a longitudinal wave. Air, being a gas, has no rigidity to sustain the sideways shearing that a transverse wave needs, so both parts of (B) hold.
- (a)Sound can travel in vacuum and it is a transverse wave in air. — Both halves are wrong: sound needs a medium and cannot travel in vacuum, and in air it is longitudinal, not transverse.
- (c)Sound can travel in vacuum and it is a longitudinal wave in air. — The 'longitudinal in air' part is right, but sound cannot travel through a vacuum because there are no particles to carry the vibration.
- (d)Sound cannot travel in vacuum and it is a transverse wave in air. — It rightly says sound cannot cross a vacuum, but sound in air is a longitudinal wave, not a transverse one.
Sound is a mechanical wave that requires a material medium (solid, liquid or gas) to propagate; it cannot travel through a vacuum. In fluids such as air it travels purely as a longitudinal wave, in which the medium's particles vibrate parallel to the direction the wave moves, producing alternating compressions and rarefactions.
The item pairs two independent facts — the need for a medium and the wave type — so a candidate must get both right. The tempting error is to reason by analogy with light, which is an electromagnetic wave and does cross a vacuum, or to loosely call sound 'transverse'.
- Sound is a mechanical wave and needs a material medium; it cannot travel through a vacuum.
- In gases and liquids sound travels only as a longitudinal wave (compressions and rarefactions).
- Light, being an electromagnetic wave, can travel through a vacuum — the key contrast with sound.
- In solids sound can travel as both longitudinal and transverse waves, but in air it is purely longitudinal.
- Assuming sound behaves like light and can cross a vacuum.
- Calling sound a transverse wave in air; in gases it is strictly longitudinal.
Asked as which paired statement about sound propagation is correct — testing both the need for a medium and the longitudinal nature of sound in air.
In which one among the following is the speed of sound maximum?
- (a) Air at 0 °C
- (b) Air at 100 °C
- (c) Water
- (d) Wood
Answer(d) Wood
Same idea that sound is a mechanical wave carried by a medium: it travels fastest in a dense solid like wood and cannot travel at all through a vacuum, which is why (B) is correct here.
Which one of the following statements about sound is NOT correct?
- (a) Sound travels at a speed slower than the speed of light
- (b) Sound waves are transverse waves
- (c) Sound waves are longitudinal waves
- (d) Sound travels faster in moist air than in dry air
Answer(b) Sound waves are transverse waves
Directly tests the same longitudinal-versus-transverse point: labelling sound 'transverse' is the wrong statement, because in air sound is a longitudinal wave — exactly the reasoning behind option (B) here.
- practice — not a real PYQ
Sound waves cannot travel through which one of the following?
- (a)Water
- (b)Steel
- (c)Vacuum
- (d)Air
Answer(c) Vacuum — sound is a mechanical wave and needs a material medium to propagate.
- practice — not a real PYQ
In air, sound propagates as a
- (a)transverse wave
- (b)longitudinal wave
- (c)electromagnetic wave
- (d)standing wave only
Answer(b) longitudinal wave — the air particles vibrate along the direction of propagation, forming compressions and rarefactions.