Question

A balloon is filled with hydrogen. For sound waves, this balloon behaves like:
A) A converging lens
B) A diverging lens
C) A concave mirror
D) Nothing can be said

Answer 1

We know that hydrogen is the lightest gas in the known world and air is the mixture of different gases. The velocity of the sound waves depends upon the type of medium.

Complete step by step solution:
We know that the density of hydrogen is less than the density of air because the air is a mixture of gases and hydrogen is the lightest gas. So, the velocity of sound in hydrogen is greater than the velocity of sound in air when the pressure is the same. This is because the molecules of hydrogen are lighter than the molecules of the air (as air is the mixture of the gases). Now, if sound waves incident on a hydrogen filled balloon, then the balloon acts like a concave lens and changes the direction of the sound waves. The balloon acts like a concave lens because the sound has greater speed in hydrogen.
The concave lens is also known as a diverging lens as it diverges the waves.
Therefore, hydrogen acts as a rarer medium due to low density and the balloon behaves as a diverging lens or concave lens.

Hence, option B is correct.

Note: In this question, we have to remember that hydrogen is a rare medium while air is a dense medium. If the medium is rare then the velocity is high and the balloon will act as a concave lens. While if the medium is dense then the velocity is low and the balloon will act as a convex lens.