g. Dispersion. The rainbow is a natural example of dispersion. As the
wavelength of light increases for a given medium, the angle of refraction
decreases.
Light waves striking droplets of water in the atmosphere are
refracted according to wavelength into colors of the spectrum. Dispersion
of these wavelengths creates one of nature's most colorful spectacles.
Figure 1-9 illustrates this dispersion.
A prism may also be used to
disperse light.
Figure 1-9.
Dispersion
h. Fluorescence.
Color can also be produced through fluorescence.
Fluorescence is the ability of a material to absorb radiations of one
wavelength and reradiate them at another wavelength, usually longer. This
is what takes place in the fluorescent lamp. The fluorescent lamp is really
a mercury vapor lamp that has been coated with a powder that fluoresces when
bombarded with ultraviolet radiation. When you apply power to the lamp, the
mercury emits ultraviolet radiations that are absorbed by the fluorescent
powder and reradiated as visible light.
The spectral quality of a
fluorescent lamp can be altered by using cadmium borate as the fluorescing
material instead of calcium tungstate. The light can be altered by adding
cadmium to the mercury emitter.
Lamps using mercury have predominantly
blue, green, and yellow spectral qualities and are not usually recommended
for color photography.
6.
Color Characteristics.
W h e n w e a s s i g n a c o l o r t o an y t h i n g , we are att e m p t i n g to de s c r i b e
c e r t a i n c o l o r ch a r a c t e r i s t i c s o f t h e ob j e c t un d e r no r m a l
1-15
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