Question

An air bubble inside water behaves like a diverging lens. Explain.

Answer 1

Air bubbles are circular in shape and we are given such an air bubble is inside water behaves like a diverging lens.
We know that the air and water are two different media with different refractive indices. The air has a refractive index of 1, whereas the water has the refractive index of 1.33.
This means water is denser medium then air, so the light rays traveling through water and entering the air bubble will bend away from the normal which can also be considered as divergent which is similar to diverging lens as shown below.
So, from this we can say that an air bubble inside the water behaves as a diverging or concave lens.