Refraction is the turning or bending of the rays of light out of their natural course.
Light, when proceeding from a luminous body--without being reflected from any opake substance or inflected by passing near one--is invariably found to proceed in straight lines without the least deviation. But if it happens to pass obliquely from one medium to another, it always leaves the direction it had before and assumes a new one. This change of direction, or _bending_ of the rays of light, is what is called _Refraction_--a term which probably had its origin from the broken appearance which a staff or a long pole exhibits, when a portion of it is immersed in water--the word, derived from the Latin _frango_, literally signifying _breaking_ or bending.
When light is thus refracted, or has taken a new direction, it then proceeds invariably in a straight line till it meets with a different medium,[7] when it is again turned out of its course. It must be observed, however, that though we may by this means cause the rays of light to make any number of angles in their course, it is impossible for us to make them describe a curve, except in one single case, namely, where they pass through a medium, the density of which either uniformly increases or diminishes. This is the case with the light of the celestial bodies, which passes downwards through our atmosphere, and likewise with that which is reflected upwards through it by terrestrial objects. In both these cases it describes a curve of the hyperbolic kind; but at all other times, it proceeds in straight lines, or in what may be taken for straight lines without any sensible error.
There are two circumstances essential to refraction. 1. That the rays of light shall pass out of one medium into another of a different density, or of a greater or less degree of resistance. 2. That they pass in an _oblique_ direction. The denser the refracting medium, or that into which the ray enters, the greater will be its refracting power; and of two refracting mediums of the same density, that which is of an oily or inflammable nature will have a greater refracting power than the other. The nature of refraction may be more particularly explained and illustrated by the following figure and description.