What would a bubble of air or an air lens in water do to parallel incoming rays of light?
An air bubble in water that is shaped like a normal glass lens would have roughly the opposite effect of the glass lens. There are at least three ways to see why.
The first way is by analogy. If we consider a convex bubble and a concave glass lens in air or vacuum, we can see that the interface between water and air in the bubble and the glass and air in the lens is really exactly the same: the material with the higher index (water or glass) curves in and the material with the lower index of refraction curves out. The only difference is that with the bubble, two of these interfaces are face to face and in the lens, they are back to back. Thus the convex bubble and the concave lens should behave the same.
The second way to analyze the problem is to do some ray tracing. A ray of light is bent toward the perpendicular when entering a higher index medium (like water or glass) and away from the perpendicular when exiting a higher index medium. For a concave bubble, a ray going through the center of the bubble will not be refracted at all at either transition. A ray perpendicular to the plane of the "lens" formed by the bubble will be refracted away from the center of the bubble as it enters the bubble and then only partially back toward the original direction as it exits the bubble. THe net effect is one of divergence.
The third method is to analyze a spherical lens using the lens-maker's formula. This will give a negative focal length for a convex lens of air inside water.
Ted Dunning, Ph.D., Chief Scientist, ID Analytics
'In a way science is a key to the gates of heaven, and the same key opens the gates of hell, and we do not have any instructions as to which is which gate.
Shall we throw away the key and never have a way to enter the gates of heaven? Or shall we struggle with the problem of which is the best way to use the key?'