A bat emits ultrasonic sound of frequency 1000 kHz in air. If the sound meets a water surface, what is the wavelength of (a) the reflected sound, (b) the transmitted sound ? Speed of sound in air is `340 ms ^(-1)` and in water `1486 ms ^(-1).`

Explanation

(i) Here sound wave reflected from water surface is travelling in air. If its wavelength is `lamda _(a)` and velocity of sound in air is `v _(a)` then,
`v _(a) = f lamda_(a) implies lamda _(a) =(v _(a))/(f )`
`= (340)/(100xx 10 ^(3)) =3.4 xx 10^(-3) m`
(ii) Here, after the sound wave is incident on the water surface, partially it gets reflected and partially it enters into water medium. Even after change in the medium, frequency of sound wave remains same (because it is source dependent and here source of sound remain same). Hence, if wavelength of sound wave in water medium is `lamda_(W)` and velocity of sound in water is `v _(W) ` then,
`v _(W) = f lamda _(W) implies lamda _(W) = (v _(W))/(f)`
`=(1486)/(100 xx 10 ^(3)) = 1.49 xx 10 ^(-2) m`