=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. [Illustration: 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´_. [Illustration: 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.