UPSC Prelims 2003 Q63: Science & Technology – Physics
Which one of the following statements is NOT correct?
- (a) The velocity of sound in air increases with the increase of temperature
- (b) The velocity of sound in air is independent of pressure
- (c) The velocity of sound in air decreases as the humidity increases
- (d) The velocity of sound in air is not affected by the change in amplitude and frequency
Answer: (c) The velocity of sound in air decreases as the humidity increases
Explanation
> Note for students: The exam asked which statement is NOT correct; the speed of sound rises with humidity, so (c) is the false statement.
Velocity of sound in air (NOT correct)
| Statement | Verdict |
|---|---|
| a. Velocity increases with temperature | Correct |
| b. Velocity is independent of pressure | Correct (at constant temperature) |
| c. Velocity decreases as humidity increases | Not correct — moist air is lighter, so the velocity increases |
| d. Velocity is not affected by amplitude and frequency | Correct |
The question asks which statement is NOT correct: it is (c), since the speed of sound increases with humidity. (The stored stem had dropped NOT; it is restored.)
Answer: (c) The velocity of sound in air decreases as the humidity increases.
Why each option is right or wrong
- (a) Incorrect. Sound travels faster in warmer air.
- (b) Incorrect. Pressure changes alone do not alter the speed.
- (c) CORRECT. The speed rises with humidity, so this statement is wrong.
- (d) Incorrect. The speed does not depend on amplitude or frequency.
Key point
v is proportional to the square root of T; humid air has lower density, so faster sound.
Remember
The speed of sound in air increases with temperature and humidity.
Common mistakes
- Reading the stem as asking for a correct statement.
- Thinking that humid air is denser than dry air.
- Believing frequency changes the speed.
Exam tip
Sound: faster in warm, humid air; faster in solids than in gases.
Sources
- NCERT Class IX Science — Sound
- NCERT Class XI Physics — Waves (speed of sound in a gas)