Question

Although an air bubble have both surfaces spherical. Inside the water it acts like concave lens.
Why???

Answer 1

The normal at any point of a spherical bubble will pass through the centre of the bubble.It is clearly visible that the air bubble acts as a concave lens in water but actually it will act like a concave lens.

Answer 2

If an air bubble is formed inside water that is spherical in shape whose surfaces are convex to water (denser medium) and concave to air (rarer medium) then the surface convex to water always diverge light on it and acts as a diverging lens. Hence, an air bubble inside water behaves as a diverging lens