1-28
together in overlapping beams of light. (NOTE: These are not the primaries used in mixing pigments,
such as in paint.) Furthermore, the COMPLEMENTARY or SECONDARY colors (magenta, yellow, and
cyan) may be shown with equal ease by mixing any two of the primary colors in overlapping beams of
light. Thus, red and green light mixed in equal intensities will make yellow light; green and blue will
produce cyan (blue-green light); and blue and red correctly mixed will produce magenta (a purplish red
light).
LIGHT AND COLOR
All objects absorb some of the light that falls on them. An object appears to be a certain color
because it absorbs all of the light waves except those whose frequency corresponds to that particular
color. Those waves are reflected from the surface, strike your eye, and cause you to see the particular
color. The color of an object therefore depends on the frequency of the electromagnetic wave reflected.
LUMINOUS BODIES
Certain bodies, such as the sun, a gas flame, and an electric light filament, are visible because they
are light sources. They are called SELF-LUMINOUS bodies. Objects other than self-luminous bodies
become visible only when they are in the presence of light from luminous bodies.
Most NONLUMINOUS bodies are visible because they diffuse or reflect the light that falls on them.
A good example of a nonluminous diffusing body is the moon, which shines only because the sunlight
falling onto its surface is diffused.
Black objects do not diffuse or reflect light. They are visible only when outlined against a
background of light from a luminous or diffusing body.
PROPERTIES OF LIGHT
When light waves, which travel in straight lines, encounter any substance, they are either
transmitted, refracted, reflected, or absorbed. This is illustrated in figure 1-19. When light strikes a
substance, some absorption and some reflection always take place. No substance completely transmits,
reflects, or absorbs all of the light rays that reach its surface. Substances that transmit almost all the light
waves that fall upon them are said to be TRANSPARENT. A transparent substance is one through which
you can see clearly. Clear glass is transparent because it transmits light rays without diffusing them (view
A of figure 1-20). There is no known perfectly transparent substance, but many substances are nearly so.
Substances through which some light rays can pass but through which objects cannot be seen clearly
because the rays are diffused are called TRANSLUCENT (view B of figure 1-20). The frosted glass of a
light bulb and a piece of oiled paper are examples of translucent materials. Substances that do not transmit
any light rays are called OPAQUE (view C of figure 1-20). Opaque substances can either reflect or absorb
all of the light rays that fall upon them.
