Diving masks often have lenses built into the glass for divers who do not have perfect vision. The lenses in a diving mask faceplate often only have a curved surfaces on the inside glass. Why is this design desirable?
I know that curving of inner surface of the lenses in the diving mask faceplate is so that diver can see clearly straight ahead while in underwater and in air. I want to know how the light travels when falling on the mask if it approaches the mask along the normal to the plane of faceplate, or at any angle $\theta$. Do we see a blurred image when the light falls at angle greater than some specific $\theta$?