11. How far from a 100-candle-power Welsbach light would the illumination be 2 foot-candles? (3) MIRRORS AND THE FORMATION OF IMAGES [Illustration: FIG. 353.--Reflection of light, (_a_) diffused, (_b_) regular.] =362. Mirrors.=--The many purposes served by mirrors in our every-day life has made their use familiar to everyone. Yet without study and experiment few understand their properties and action. _Any smooth_ surface may serve as a mirror, as that of glass, water, polished wood, or metal. Most objects, unlike mirrors, have irregular surfaces; these scatter or diffuse the light that falls upon them. (See Fig. 353_a_.) This is called _diffused or irregular reflection_. The reflection of light from the smooth surface of a mirror is _regular_. (See Fig. 353_b_.) In every case of reflected light, however, the angle of reflection equals the angle of incidence, diffusion being due to the irregularity of the surface. It is by means of the light "diffused" from the surface of illuminated bodies, such as plants, animals, food, and manufactured articles, that we "see" the various objects about us, and it is this light that enables us to judge of their distance, size, form, color, etc. The moon is seen by the sunlight reflected from its surface. Moonlight is therefore sunlight diffused by reflection. The _new moon_ is that phase or condition of the moon when only a narrow strip of the moon's illuminated surface is turned toward the earth. At the time of the _full moon_ the whole illuminated surface is seen. =363. Images Formed by a Plane Mirror.=--The most common use of mirrors is in the formation of images. The way in which images are formed by a plane mirror may be illustrated by diagrams. Thus in Fig. 354, let _L_ represent a luminous body and _E_ and _E´_ two positions of the observer's eye. Take any line or ray as _LO_ along which the light from _L_ strikes the mirror _O-O´_. It will be reflected so that angle _LOP_ equals angle _POE_. Similarly with any other ray, as _LO´_, the reflected ray _O´E´_ has a direction such as that angle _L´O´E´_ equals angle _P´O´E´_. Any other rays will be reflected in a similar manner, each of the reflected rays appearing to the eye to come from a point _L´_ behind the mirror. [Illustration: FIG. 354.--The virtual image of a fixed object as seen in a plane mirror, has the same location from every position of the observer's eye.]