A wave that passes all the way through a piece of material with parallel sides leaves the material at the same angle that it entered. The wave un-bends when it exits the material by the same amount th...A wave that passes all the way through a piece of material with parallel sides leaves the material at the same angle that it entered. The wave un-bends when it exits the material by the same amount that it bent when entering but this is only true if the sides of the material are parallel. Convex and concave lenses have sides that are not parallel (except near the center). In this case parallel rays of light end up exiting in different directions.
