Question

A plane wave of sound travelling in air is incident upon a plane water surface. The angle of incidence is ${60}^{\circ }$. If velocity of sound in air and water are 330 m/s and 1400 m/s, then the wave undergoes

• A. refraction only
• B. reflection only
• C. Both reflection and refraction
• D. neither reflection nor refraction

Answer 1

Answer: (B)

reflection only

Given: Angle of incidence in air $i={60}^o$

Some part of the wave will surely get reflected when it strikes the plane surface.

Let also some part of wave will refract and $r$ be the angle of refraction.

Using $\frac{sinr}{sini}=\frac{v_w}{v_a}$ for refraction from air to water.

$\frac{sinr}{sin{60}^o}=\frac{1400}{330}⟹sinr=3.67$ which is $>1$

As $sin\theta$ can never be greater than $1$, thus refraction is not possible.

Hence the wave will only reflect.