4. Light Sources. To see, we must have a source of light, just as in the process
of hearing we must have a source of sound before we can hear.
When we speak of
light, we usually think of light coming from the sun or the light which is emitted
from some artificial lighting source such as electrical light. This type of light
is referred to as direct light.
Another type of light is indirect or reflected
light, given off by an object when direct light strikes it. Direct light falling
on an object is either absorbed or reflected. If all the light is reflected, the
object appears white.
If direct light is entirely absorbed, the object appears
black. The larger the amount of light that is reflected by an object, the brighter
the object will appear to the eye. In addition, the brighter the light source, the
brighter the object will become. This can be demonstrated by casting a shadow on a
portion of an object and noting the difference in brightness of the two areas. The
portion without the shadow will appear brighter.
5. The visible light spectrum. Light is one of many forms of radiant energy. Any
energy that travels by wave motion is considered radiant energy.
Classified in
this group along with light are sound waves, X-rays, and radio waves. As shown in
Figure 1-1, light which is useful to the eye occupies only a small portion of the
radiant energy spectrum. Sound is located at the lower end of the spectrum; cosmic
rays are at the upper end.
Light falls just beyond the middle of the spectrum.
Along the top of the spectrum is the frequency scale, and along the bottom is the
Angstrom unit scale (fig 1-1).
Figure 1-1.
Radiant energy spectrum
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