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
Correct answer
Explanation
The correct answer is B, Sound waves are transverse waves. This statement is false, because sound in air travels as longitudinal waves, not transverse waves.
In a longitudinal wave, the particles of the medium vibrate back and forth along the direction in which the wave moves, forming compressions and rarefactions. Sound needs a medium and cannot travel through a vacuum. In air at 20 °C it moves at about 343 m/s, while light moves at about 3 × 10⁸ m/s, which is why we see lightning before we hear thunder. Moist air is less dense than dry air at the same temperature and pressure, because water vapour is lighter than nitrogen and oxygen, so sound travels a little faster in it.
A is wrong as a choice because it is a true statement: sound is far slower than light. C is wrong as a choice because sound waves in air are indeed longitudinal. D is wrong as a choice because sound really does travel faster in moist air.
Exam tip: sound is longitudinal and needs a medium; speed rises with temperature and humidity; light is transverse and crosses a vacuum.
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