8. Try to read a printed page by looking at its image in a mirror. write your name backward on a sheet of paper, and then look at the image of the writing in a mirror. What effect is produced by the mirror in each case? 9. If the point of a pencil is held to the surface of a piece of plate-glass mirror two or more images may be seen in the mirror. Explain. 10. Given a small lighted candle, a concave mirror, a meter stick, and a white screen, how would you prove the statements made in Arts. 369 and 370 concerning the location of images formed by concave mirrors? Make the diagram in each case. 11. Why do images seen in a quiet pond of water appear inverted? Explain by a diagram. (4) REFRACTION OF LIGHT =375. Common Examples of Refraction.=--Everyone has noticed the apparent bending of an oar, of a stick, or of a spoon when placed in water (see Fig. 366), while many have observed that the bottom of a pond or stream looks nearer to the surface than it really is. These and similar illusions are due to the _refraction_ or bending of light rays as they pass from one medium to another. The principles of refraction are among the most useful found in the study of light since application is made of them in the construction and use of important optical instruments, such as the camera, microscope, telescope, and the eye. [Illustration: FIG. 366.--The stick appears to be bent on account of refraction.] =376. Action of Light Undergoing Refraction.=--If a beam of sunlight be admitted to a darkened room and reflected by a mirror so that it strikes the surface of water in a glass jar, a part of the beam may be seen to be reflected while another portion is transmitted through the water (Fig. 367). The reflected beam follows the law of reflection while the transmitted beam is seen to be _refracted_, or to have its courses slightly changed in direction upon entering the water. If the mirror is turned so that the angle at which the light strikes the water is changed, the amount of refraction or change of course of the light is varied. When the light strikes the water perpendicularly there is no refraction. On the other hand, the greater the angle at which the light strikes the water the greater the bending. [Illustration: FIG. 367.--Part of the ray is reflected and part passes into the water and is refracted.] [Illustration: FIG. 368.--Illustrating the laws of refraction of light.] =377. Laws of Refraction.= The action of light on entering, passing through, and leaving a great variety of substances has been carefully studied. A summary of the results of these observations is given in the following _laws of refraction_: I. _When light enters a transparent body, perpendicularly, it passes on without changing its direction._ II. _When light enters a denser transparent body obliquely, it is bent toward the perpendicular; when light enters a less dense body obliquely, it is bent away from the perpendicular._ (See Fig. 368.)