Although the ratio of the sines is constant, the refractive index varies in different media. Thus that of air is 1·0003; of water, 1·336; of Canada balsam, 1·549; of crown glass, from which window-panes are made, 1·535; of flint glass, from which bottles are made, 1·6; of Faraday’s heavy glass, composed of silicated borate of lead, 1·8; and of that consisting of borate of lead, 2·0.
A knowledge of this “law of the sines” is of practical importance in determining the direction which the rays will pursue when transmitted through glass lenses, &c. the refractive index of which is known; or in ascertaining the curve which should be given to their surfaces for producing a particular refraction and focal length. Thus, supposing the plate of glass in [Pl. I.] fig. 2 to consist of crown glass, the refractive index of which is 1·5, the length of the sine of refraction, t r, will be equal to one part or dimension, while the sine of incidence, s i, is equal to one part and a half.
It must be remarked that when light is incident at a right angle to the surface of the medium, no refraction takes place, the transmitted ray pursuing its original course.
When a ray of light leaves a denser medium, such as glass, to enter a rarer medium, such as air, it becomes refracted from the perpendicular. In such case, the angle of refraction being greater than the angle of incidence, its sine will also be greater than that of the latter; but the ratio is still preserved.
Reflexion.—When rays of light fall upon a plane surface, as the flat surface of the mirror, a greater or less number of them are reflected, and this according to a definite law, by which the angle of incidence, or that formed by the incident ray with the perpendicular, is equal to the angle of reflexion, or that formed by the reflected ray with the same. Thus, as shown in [Pl. XII.] fig. 3, the angle i b p, formed by the incident ray i b with the perpendicular p, is equal to the angle p b r, formed by the reflected ray b r with the perpendicular p b.