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Candlepower and Footcandles 

Illumination is the light received from a light source. The intensity of illumination is measured in footcandles. A footcandle 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

Figure 5-3-3.–Light rays reflected, absorbed, and refracted.

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. 5-3-4). 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


Figure 5-3-4.–Terms used to describe the reflection of light.

the angle of incident. 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 incident is equal to the angle of reflection.

Reflection from a smooth-surfaced object presents a few problems. It is a different matter, however, when a rough surface reflects the light. The law of reflection still holds out because the surface is uneven, the angle of incident is different for each ray of light. The reflected light is scattered in all directions as shown in figure 5-3-5 and is called irregular or diffused light.

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