Physics
A girl claps in a classroom but is unable to hear an echo. Which option best explains the reason for the same ?
- There is no reflection of sound from the walls.
- Sound waves are completely absorbed by the walls.
- Echo returns in less than 0.1 s.
- Echoes require sounds of ultrasonic frequency.
Answer
Echo returns in less than 0.1 s.
Reason — When the girl claps in the classroom, the sound does get reflected from the walls, but the reflected sound reaches her ears in a very short time—less than 0.1 seconds.
For an echo to be heard distinctly, there must be a minimum time gap of about 0.1 s between the original and reflected sound and since the classroom walls are too close, this condition is not satisfied, so the reflected sound merges with the original sound and no separate echo is heard.
Related Questions
Infrasonic sound waves have a frequency :
- between 20 Hz to 10,000 Hz
- below 20 Hz
- above 20,000 Hz
- between 20 Hz to 20,000 Hz
The diagram shown below depicts a displacement time graph of particle a wave travelling with speed 20 m/s.
The frequency of the wave is :
- 50 Hz
- 33.3 Hz
- 100 Hz
- 1000 Hz
In ancient India, dome shaped courts were built in forts to allow even the faintest of sounds to be heard from far away. Which statement best explains this effect?
- Dome increases the frequency of the sound so it becomes louder.
- Dome absorbs high frequency sound and re-emits it at a high amplitude.
- Dome focuses the reflected sound and multiple reflected waves reinforce at listener's position.
- Dome traps air thereby increasing air pressure resulting in amplification of sound.
Assertion (A): Sound waves can travel in vacuum, but light waves cannot.
Reason (R): Light is an electromagnetic wave, but sound is a mechanical wave .
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true and R is not the correct explanation of A.
- assertion is false but reason is true.
- assertion is true but reason is false.