=378. The cause of refraction= may be illustrated by considering a line of men moving across a field and occupying at equal time intervals the successive positions 1, 2, 3, etc., indicated in Fig. 369. Suppose that the upper and lower parts of the field have a smooth hard surface, while at the center is a strip of newly ploughed ground. The line will move more slowly over the ploughed field than over the hard field. This will result in a retardation of the end of the line first striking the soft ground with a resulting change of direction of the line, _toward_ the perpendicular to the edge of the field (_on entering the place of more difficult travel_), and _away_ from the perpendicular on moving into a place where _increased speed results_. [Illustration: FIG. 369.--Diagram illustrating the cause of refraction.] =379. Index of Refraction.=--By studying the change of direction of the marching men as shown in Fig. 369 it is evident _first_ that it is due to a difference in speed in the two media. It is not easy to measure the speed of light in a medium. However, the amount of refraction may be determined easily and from this the _relative_ speed may be computed. The _number that expresses the ratio of the speed of light in air to its speed in another medium is called the index of refraction of that medium_. The relative speeds of light, or the indices of refraction for some substances, are: water, 1.33, crown glass, 1.51, flint glass, 1.61, diamond, 2.47, carbon bisulphide, 1.64. [Illustration: FIG. 370.--The incident ray and the emergent rays are parallel.] =380. Plates, Prisms, Lenses.=--The refraction of light is usually observed when it is passing through a plate, a prism, or a lens. The important differences between the effects of each in refracting light are illustrated in Figs. 370, 371 and 372. In Fig. 370 it is seen that the refraction of the ray on entering the glass is counteracted by the refraction away from the perpendicular upon leaving it. So that the entering and emergent rays are _parallel_. In Fig. 371 the refraction at the two surfaces of the prism results in a change of direction of the ray, the course being bent toward the _thicker_ part of the _prism_. In Fig. 372 it may be noticed that the convex lens resembles two prisms with their bases together. Since all parts of the lens refract light toward the thicker part, the center, the effect of the convex lens is to bring the rays of light to a focus, at _F_. [Illustration: FIG. 371.--Effect of a prism upon a ray of light.] [Illustration: FIG. 372.--The convex lens brings the rays of light to a focus.]