water, is termed the normal, and a ray of light passed in this direction will continue in a straight line to the point E. If, however, the ray is passed in an oblique direction, such as ND, it will be seen that the ray is bent or refracted in the direction DM; if the ray of light is passed in any other oblique direction, such as JD, the refracted ray will be in the direction DK. The angle NDC is called the angle of incidence and MDE the angle of refraction. If we measure accurately the line NC, we shall find that it is 11/3, or more exactly 1.336, times greater than the line EM. If we repeat this measurement with the lines JH and PK we shall find that the line JH also bears the proportion of 1.336 to the line PK. The line NC is called the sine of the angle of incidence NDC, and EM the sine of the angle of refraction MDE.
Therefore in water the sine of the angle of incidence is to the sine of the angle of refraction as 1.336 is to 1, and this is true whatever the position of the incident ray with respect to the surface of the water. From this we say that the sines of the angles of incidence and refraction have a constant proportion or ratio to one another.
The number 1.336 is termed the refractive index, or coefficient, or the refractive power of water. The refractive power varies, however, with other fluids and solids, and a complete table will be found in any good work on optics.
Glass is the substance most commonly used for refracting the rays of light in optical work, the glass being worked up into different forms according to the purpose for which it
is intended. Solids formed in this way are termed lenses. A lens can be defined as a transparent medium which, owing to the curvature of its surfaces, is capable of converging or diverging the rays of light passed through it. According to its curvature it is either spherical, cylindrical, elliptical, or parabolic. The lenses used in optics are always exclusively spherical, the glass used in their construction being either crown glass, which is free from lead, or flint glass, which contains lead and is more refractive than crown glass. The refractive power of crown glass is from 1.534 to 1.525, and of flint glass from 1.625 to 1.590. Spherical surfaces in combination with each other or with plane surfaces give rise to six different forms of lenses, sections of which are given in Fig. 65.
All lenses can be divided into two classes, convex or converging, or concave or diverging. In the figure, b, c, g are converging lenses, being thicker at the middle than at the borders, and d, e, f, which are thinner at the middle, being diverging lenses. The lenses e and g are also termed meniscus lenses, and a represents a prism. The line XY is the axis or normal of these lenses to which their plane surfaces are perpendicular.
Let us first of all notice the action of a ray of light when passed through a prism. The prism, Fig. 66, is represented by the triangle BBB, and the incident ray by the line TA.
Where it enters the prism at A its direction is changed and it is bent or refracted towards the base of the prism, or towards the normal, this being always the case when light passes from a rare medium to a dense one, and where the light leaves the opposite face of the prism at D it is again refracted, but away from the normal in an opposite direction to the incident ray, since it is passing from a dense to a rare medium. The line DP is called the emergent or refracted ray. If the eye is placed at T, and a bright object at P, the object is seen not at P, but at the point H, since the eye cannot follow the course taken by the refracted rays. In other words, objects viewed through a prism always appear deflected towards its summit.