Bodies which suffer the rays of light to pass through them, such as
air, water, or glass, are called refracting media. When rays of light
enter these, they do not proceed in straight lines, but are said
to be refracted, or bent out of their course. But if the ray falls
perpendicularly on the glass, there is no refraction, and it proceeds
in a direct line; hence, refraction only takes place when rays fall
obliquely or aslant on the media.