Question

An air bubble inside water behaves like a _________ lens.

Answer 1

Concave Lens:

1. It is a lens that diverges a light beam that falls on it.
2. It has at least one surface that is curved inside.

Air Bubble:

1. These are circular spheres made up of thin water films and contain air.
2. The surface of an air bubble in water bulges outwards due to the air pressure inside the bubble.

How does an air bubble act like a concave lens:

1. The refractive index of water is greater than that of air, ${\eta }_{\mathrm{water}}>{\eta }_{\mathrm{air}}$. This implies that, water is a denser medium than air.
2. Let us consider that a light ray passes through water and enters an air bubble. So, the light ray enters from a denser to rarer medium. So, a ray diagram can be drawn as given below.
3. The ray of light entering the air bubble will diverge, since the ray of light entering from a denser to rarer medium bends away from the normal.
4. Hence, it behaves like a concave lens.

Hence, the air bubble inside water behaves like a concave lens.