Question

How can you apply lens makers formula to determine nature of air bubble in water

Answer 1

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. The normal at any point of a spherical bubble will pass through the center of the bubble. It is clearly visible that the air bubble acts as a diverging lens (concave lens) in water.