What should be the minimum distance between the source and reflector in water so that the echo is heard distinctly? (The speed of sound in water = 1400 ms^-1).

As we know,

$\text {Speed of sound (V)} = \dfrac{\text{Total distance travelled (2d)}}{\text {time interval (t)}} \\[0.5em]$

Total distance travelled by the sound in going and then coming back = 2d

Given,

V = 1400 ms^-1

As important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 s after the original sound dies off.

Hence, t = 0.1 s

Substituting the values in the formula above, we get,

$1400 = \dfrac{2d}{0.1} \\[0.5em] \Rightarrow 2d = 1400 \times 0.1 \\[0.5em] \Rightarrow d = \dfrac{140}{2} \\[0.5em] \Rightarrow d = 70 m \\[0.5em]$

Hence, the minimum distance between the source and reflector in water so that echo is heard distinctly = 70 m