Any body, which is so constituted as to yield a passage to the rays of light, is called a MEDIUM. Air, water, glass, &c. are mediums of light. If any medium afford an easy passage to the rays of light, it is called a RARE MEDIUM; but if it do not afford an easy passage to these rays, it is called a DENSE MEDIUM.
Let Z, Fig. 2. Plate I. be a rare medium, and Y a dense one; and let them be separated by the plane surface G H. Let I K be a perpendicular to it, and cutting it in C.
With the center C, and any distance, let a circle be described. Then let A C be a ray of light, falling upon the dense medium. This ray, if nothing prevented, would go forward to L; but because the medium Y is supposed to be denser than Z, it will be bent downward toward the perpendicular I K, and describe the line C B.
The ray A C is called the INCIDENT RAY; and the ray C B, the REFRACTED RAY.
The angle A C I is called the ANGLE OF INCIDENCE, and the angle B C K is called the ANGLE OF REFRACTION.
If from the point A upon the right line C I, there be let fall the perpendicular A D, that line is called the sine of the angle of incidence.
In the same manner, if from the point B, upon the right line I K, there be let fall the perpendicular B E, that line will be the sine of the angle of refraction.
The sines of the angles are the measures of the refractions, and this measure is constant; that is, whatever is the sine of the angle of incidence, it will be in a constant proportion to the sine of the angle of refraction, when the mediums continue the same. A general idea of refraction may be formed from the following experiments.
EXPERIMENT 1. Let A B C D, Fig. 3. Plate I. represent a vessel so placed, with respect to the candle E, that the shadow of the side A C may fall at D. Suppose the vessel to be now filled with water, and the shadow will withdraw to d; the ray of light, instead of proceeding to D, being refracted or bent to d. And there is no doubt but that an eye, placed at d, would see the candle at e, in the direction of the refracted ray d A. This is also confirmed by the following pleasing experiment.
2. Lay a shilling, or any piece of money, at the bottom of a bason; then withdraw from the bason, till you lose sight of the shilling; fill the bason nearly with water, and the shilling will be seen very plainly, though you are at the same distance from it.