Question

inside water air bubbles behave like concave lens why give reason​

Answer 1

It is clearly visible that the air bubble acts as a DIVERGING LENS (concave lens) in water. ... Since air is optically rarer than water, the ray of light bends away from the normal at the point of incidence when moving from water to air while it bends towards the normal while moving from air to water.

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Answer 2

when a ray of light  strikes the bubble in water , light enter from denser medium [water] to rarer medium[air] it bends towards the normal and again at the other end refraction takes place that is light ray will enter from air [rare] to denser [water] and it moves away from normal.

Thus we can see that air bubble in water acts like concave lens / Diverging Lens.

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