An air bubble in water will act like a

An air bubble in water acts as a concave lens. An air bubble has a spherical shape that is convex in appearance. However, its behavior depends on the refractive index of the mediums involved. Water is optically denser than air. When light rays travel from water into the air bubble, they move from a denser medium to a rarer medium and then back into water. Because the refractive index of the air inside the bubble is less than the refractive index of the surrounding water, the rays of light diverge as they pass through. A lens that causes parallel rays of light to diverge is a concave lens, also known as a diverging lens. Therefore, even though the bubble is physically shaped like a convex sphere, it functions optically as a concave lens.