364. Light Waves and Wave Diagrams.—Just as a stick continually moved at the surface of a body of water sets up a series of waves spreading in all directions, so one may imagine a train of waves sent out by a luminous body L (as in Fig. 355) to the mirror MN. These waves will be reflected from the mirror as if the source of light were at L´. It is much simpler and more convenient to locate the position of the image of a point by the use of lines or "rays" (as in Fig. 354) than by the wave diagram (as in Fig. 355). In all ray diagrams, however, it should be kept in mind that the so-called ray is a symbol used to represent the direction taken by a part of a light wave. Thus in Fig. 354, the light from L moving toward O is reflected to E along the line OE, the heavy lines representing rays.
Fig. 355.—Wave diagram of image formed in a plane mirror.
365. To locate the image of an object formed by a plane mirror requires simply an application of the law of reflection. Thus in Fig. 356 let AB represent an object and MN a plane mirror. Let AA´ be a ray from A striking the mirror perpendicularly. It is therefore reflected back along the same line toward A. Let AO represent any other ray from A. It will be reflected along OE so that angle r equals i. The intersection of AC and OE at A´ behind the mirror locates the image of the point A, as seen by reflection from the mirror. The triangles ACO and A´CO may be proved equal by geometry. Therefore A´C equals AC. This indicates that the image of a point formed by a plane mirror is the same distance back of the mirror as the point itself is in front of it. This principle may be used in locating the image of point B at B´. Locating the position of the end points of an image determines the position of the whole image as A´B´.
Fig. 356.—The image A´B´ is as far back of the mirror M N as the object A B is in front of the mirror.
366. How the Image is Seen.—Suppose the eye to be placed at E. It will receive light from A by reflection as if it came from A´. Similarly light starting from B reaches the eye from the direction of B´. There is nothing back of the mirror in reality that affects our sight, the light traveling only in the space in front of the mirror. Yet the action of the reflected light is such that it produces the same effect as if it came from behind the mirror. Images such as are seen in plane mirrors are called virtual to distinguish them from real images, in which light actually comes to the eye from the various parts of the visible image, as from the real image formed by a projecting lantern upon a screen, or by an aperture as in the pin-hole camera. Real images therefore are those that can be obtained upon a screen while virtual images cannot.
367. Multiple Reflection.—If the light from an object is reflected by two or more mirrors various effects may be produced, as may be illustrated by the kaleidoscope. This consists of three plane mirrors so arranged that a cross-section of the three forms an equilateral triangle. The mirrors are placed in a tube across the end of which is a compartment with a translucent cover containing pieces of colored glass. On looking through the tube, the reflections from the several surfaces produce beautiful hexagonal designs.
Fig. 357.—Perspective view of "Pepper's ghost."
Fig. 358.—Diagram of the "Pepper Ghost" illusion.