A plane wave of sound traveling in air is incident upon a plane surface of a liquid. The a

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14.Waves and Sound

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A plane wave of sound traveling in air is incident upon a plane surface of a liquid. The angle of incidence is $60^o.$ The speed of sound in air is $300 \,m /s$ and in the liquid it is $600\, m /s .$ Assume Snell’s law to be valid for sound waves.

A

The wave will refract into liquid away from normal

B

The wave refract into liquid towards the normal

C

The wave will reflect back into air

D

none of these

Solution

Since $\mathrm{V}_{\text {liquid }}>\mathrm{V}_{\text {Air }}$

i.e. liquid is rarer than air for sound waves

Angle of total internal reflection (critical angle)

$=\sin ^{-1}\left(\frac{\mathrm{v}_{\mathrm{liq}}}{\mathrm{v}_{\mathrm{air}}}\right)=\sin ^{-1}\left(\frac{1}{2}\right)=30^{\circ}$

Hence sound wave is totally reflected back.

Standard 11

Physics

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