A student wishes to measure the speed of sound in air. He plans to measure the time between making the sound and hearing its echo from a cliff. What type of sound should he make and which distance should he use in his calculations ?

	Sound	Distance
(a)	Continuous sound	$\dfrac{\text{Distance to cliff}}{2}$
(b)	Continuous sound	Distance to cliff x 2
(c)	Short, sharp sound	$\dfrac{\text{Distance to cliff}}{2}$
(d)	Short, sharp sound	Distance to cliff x 2

1. Option (a)
2. Option (b)
3. Option (c)
4. Option (d)

Option (d)

Reason — To measure the speed of sound using echo, the student must make a short, sharp sound so that the reflected sound (echo) can be clearly distinguished and the sound travels to the cliff and back, so the total distance travelled by the sound is 2 × the distance to the cliff.