105. Why the Image seems to be behind the Mirror. If a candle is placed in front of a mirror, as in Figure 62, one of the rays of light which leaves the candle will fall upon the mirror as _AB_ and will be reflected as _BC_ (in such a way that the angle of reflection equals the angle of incidence). If an observer stands at _C_, he will think that the point _A_ of the candle is somewhere along the line _CB_ extended. Such a supposition would be justified from Section 102. But the candle sends out light in all directions; one ray therefore will strike the mirror as _AD_ and will be reflected as _DE_, and an observer at _E_ will think that the point _A_ of the candle is somewhere along the line _ED_. In order that both observers may be correct, that is, in order that the light may seem to be in both these directions, the image of the point _A_ must seem to be at the intersection of the two lines. In a similar manner it can be shown that every point of the image of the candle seems to be behind the mirror. [Illustration: FIG. 62.--The image is a duplicate of the object, but appears to be behind the mirror.] It can be shown by experiment that the distance of the image behind the mirror is equal to the distance of the object in front of the mirror. 106. Why Objects are Visible. If the beam of light falls upon a sheet of paper, or upon a photograph, instead of upon a smooth polished surface, no definite reflected ray will be seen, but a glare will be produced by the scattering of the beam of light. The surface of the paper or photograph is rough, and as a result, it scatters the beam in every direction. It is hard for us to realize that a smooth sheet of paper is by no means so smooth as it looks. It is rough compared with a polished mirror. The law of reflection always holds, however, no matter what the reflecting surface is,--the angle of reflection always equals the angle of incidence. In a smooth body the reflected beams are all parallel; in a rough body, the reflected beams are inclined to each other in all sorts of ways, and no two beams leave the paper in exactly the same direction. [Illustration: FIG. 63.--The surface of the paper, although smooth in appearance, is in reality rough, and scatters the light in every direction.] Hot coals, red-hot stoves, gas flames, and candles shine by their own light, and are self-luminous. Objects like chairs, tables, carpets, have no light within themselves and are visible only when they receive light from a luminous source and reflect that light. We know that these objects are not self-luminous, because they are not visible at night unless a lamp or gas is burning. When light from any luminous object falls upon books, desks, or dishes, it meets rough surfaces, and hence undergoes diffuse reflection, and is scattered irregularly in all directions. No matter where the eye is, some reflected rays enter it, and the various objects are clearly seen. CHAPTER XI REFRACTION