Characteristics When  light  waves  encounter  any  substance,  they are  either  reflected,  absorbed,  or  refracted.  (See  fig. 5-9.)   Substances that permit the penetration of clear vision through them and transmit almost all the light falling upon them, such as glass and air, are transparent. There    is    no    known    substance    that    is    perfectly transparent, but many are nearly so. Those substances that allow the passage of part of the light but appear clouded and impair vision substantially, such as frosted light     bulbs,     are     considered     translucent.     Those substances  that  do  not  transmit  any  light  are  termed opaque. All  objects  that  are  not  light  sources  are  visible only because they reflect all or some part of the light reaching  them  from  a  luminous  source.  If  light  is neither refracted nor reflected, it is absorbed or taken up by the medium. When light strikes a substance, some absorption and some reflection always takes place. No substance  completely  refracts  (transmits),  reflects,  or absorbs all the light that reaches its surface. Figure 5-9 illustrates this refraction, absorption, and reflection of light using a flat pane of glass. Candlepower and Foot-candles Illumination   is   the   light   received   from   a   light source.  The  intensity  of  illumination  is  measured  in foot-candles.   A   foot-candle   is   the   amount   of   light falling  upon  a  1-square-foot  surface,  which  is  1  foot away from a 1-candlepower light source. REFLECTION The term reflected light refers to those light waves that are neither transmitted nor absorbed but are thrown back from the surface of the medium they encounter. If a ray of light is directed against a mirror, the light ray that strikes the surface is called the incident ray; the one that bounces off is the reflected ray (see fig. 5-10). The imaginary line perpendicular to the mirror at the point where the ray strikes is the normal. The angle between the   incident   ray   and   the   normal   is   the   angle   of incidence. The angle between the reflected ray and the normal is the angle of reflection. If  the  surface  of  the  medium  contacted  by  the incident  light  ray  is  smooth  and  polished,  such  as  a mirror,  the  reflected  light  is  thrown  back  at  the  same angle to the surface as the incident light. The path of the light reflected from the surface forms an angle exactly equal  to  the  one  formed  by  its  path  in  reaching  the medium. This conforms to the law of reflection, which states that the angle of incidence is equal to the angle of reflection. Reflection from a smooth-surfaced object presents few problems. It is a different matter, however, when a rough surface reflects light. The law of reflection still holds  but  because  the  surface  is  uneven,  the  angle  of incidence is different for each ray of light. The reflected light  is  scattered  in  all  directions  as  shown  in  figure 5-11 and is called irregular or diffused light. 5-15 INCIDENT LIGHT RAYS REFLECTED RAYS ABSORBED RAYS REFRACTED RAYS AG5f0509 Figure 5-9.—Light rays reflected, absorbed, and refracted. INCIDENT RAY PERPENDICULAR MIRROR REFLECTED RAY ANGLE OF INCIDENCE ANGLE OF REFLECTION NORMAL OR AG5f0510 Figure 5-10.—Terms used to describe the reflection of light. AG5f0511 INCIDENT LIGHT INCIDENT LIGHT REFLECTED LIGHT SPECULAR DIFFUSED REFLECTED LIGHT SCATTERED (A) (B) Figure 5-11.—Reflected light. (A) Regular (specular); (B) Irregular (diffused).