GEOMETRICAL OPTICS.
It is assumed in this subject—as the result either of direct experiments or of deduction from physical optics—that 'rays of light' pass in straight lines through any homogeneous medium that they are independent of each other: that when a ray meets a surface separating two trans parent material media, e.g. water and air, it produces a reflected ray back into the first medium and a refracted ray in the second medium, the two having definite directions given by the laws of reflection and refraction. The laws of reflection are: If a to plane is drawn to the separating surface at the point where the ray strikes, the line perpentlicular to this plane at the, point bisects the angle between the incident and reflected rays and lie-, in the plane which includes them. The laws of ordinary refraction are: It' a tangent plane is drawn to the separating surface at the point where the ray strikes and if a perpendicular line to this plane is drawn through this point, it will lie in the plane including the incident and refracted rays; and if the angles these rays and the perpendicular line are 0, and 0„ the ratio of sin 0, to sin 0, is a constant for the two media for a given kind of light. TIlis ratio is
called the 'index of refraction of the second medium with reference to the first ;' it is found to vary for light of different colors. If the first medium is the pure ether, this ratio is called the index of refraction of the second medium.
There are certain bodies—either crystal or isotropic bodies in a state of strain—such that when an incident ray falls upon them, not one, but in general tiro refracted rays are produced. This is called 'double refraction.' In one class of bodies one of the refracted rays obeys the ordinary laws of refraction and is called the 'ordinary ray,' while the other ray does not in general do so and 'is called the 'extraordinary' ray. In other bodies neither of the refracted rays obeys the ordinary laws in general. The main features of bodies of the former class. e.g. Iceland spar, were fully explained by Huygens on the wave theory, while those of the latter class, e.g. aragonite, were explained similarly by Fresnel. See section on Physical optics below.