Does an air bubble in water act as convex or a concave lens?
The principle behind the working of a lens made of glass is the change in the path of light at the glass-air interface. The refractive index of air is approximately 1 and that of glass is typically around 1.5.
In the case of an air bubble in water, the refractive index of water is more than that of air. Here again there is a change in the path of light at the interface between air and water.
Now, an air bubble is spherical in shape. This makes it equivalent to two convex interfaces meeting each other with light entering from a medium with a higher refractive index. This behaves in a way that is equivalent to two concave interfaces if the light were entering from a medium with a lower refractive index into one with a higher refractive index.
Therefore an air bubble in water acts in the same manner as a concave lens made of glass does in air.