Let ADHI fig. 2, be a body of water, AD its surface, C a point in which a ray of light BC enters from the air into the water. This ray, by the greater density of the water, instead of passing straight forward in its first direction to K, will be bent at the point C, and pass along in the direction CE, which is called the _refracted_ ray. Let the line FG be drawn perpendicular to the surface of the water in C, then it is evident that the ray BC, in passing out of air, a _rare_ medium, into a _dense_ medium, as water, is refracted into a ray CE which is _nearer_ to the perpendicular CG than the incident ray BC, and on the contrary, the ray EC passing out of a denser medium into a rarer will be refracted into CB, which is _farther_ from the perpendicular.
The same thing may be otherwise illustrated as follows:--suppose a hole made in one of the sides of the vessel as at _a_, and a lighted candle placed within two or three feet of it, when empty, so that its flame may be at L, a ray of light proceeding from it will pass through the hole _a_ in a straight line LBCK till it reach the bottom of the vessel at K, where it will form a small circle of light. Having put a mark at the point K, pour water into the vessel till it rise to the height AD, and the round spot that was formerly at K, will appear at E; that is, the ray which went straight forward, when the vessel was empty, to K, has been bent at the point C, where it falls into the the water, into the line CE. In this experiment it is necessary that the front of the vessel should be of glass, in order that the course of the ray may be seen; and if a little soap be mixed with the water so as to give it a little mistiness, the ray CE will be distinctly perceived. If, in place of fresh water we pour in salt water, it will be found that the ray BC is more bent at C. In like manner alcohol will refract the ray BC more than salt water, and oil more than alcohol, and a piece of solid glass, of the shape of the water, would refract the light still more than the oil.
The angle of refraction depends on the obliquity of the rays falling on the refracting surface being always such, that the sine of the incident angle is to the sine of the refracted angle, in a given proportion. The _incident_ angle is the angle made by a ray of light and a line drawn perpendicular to the refracting surface, at the point where the light enters the surface. The _refracted_ angle is the angle made by the ray in the refracting medium with the same perpendicular produced. The _sine_ of the angle is a line which serves to measure the angle, being drawn from a point in one leg perpendicular to the other. The following figure (fig. 3.) will tend to illustrate these definitions.