The time period of a sound wave is 2.5 × 10⁻³ s. Its frequency is:
1 250 Hz
2 320 Hz
3 400 Hz
4 500 Hz

Correct Answer: (c)
Explanation
The relationship between frequency and time period is:
f = 1/T
Given,
Time period, T = 2.5 × 10⁻³ s
Therefore,
f = 1 / (2.5 × 10⁻³}
= (1 / 2.5} × 10³
= 0.4 × 1000
= 400 Hz
Hence, the correct answer is Option (3}: 400 Hz.
Additional Information
• Frequency is the number of vibrations produced in one second.
• SI unit of frequency = Hertz (Hz}.
• Time period and frequency are inversely proportional:
· f = 1/T
· T = 1/f
• Human audible range: 20 Hz – 20,000 Hz (20 kHz}.