Sources of light
Anyone who has spent a moonless night in the country, in a forest,
or at sea knows how very dark it can be when the sun is on the other
side of the earth. At early dawn, objects that we could not see a few
minutes earlier begin to take shape. Then details sharpen, colors appear
and brighten, and daylight begins. It is the light from the sun, rising
above the horizon in the east, that gives shape, detail, and color to our world.
The sun, the stars, lamps, even lightning bugs, give off light. They
are called luminous bodies (from the Latin word lumen, meaning light).
Other objects - trees, grass, your keyboard, for example - are nonluminous.
They are visible only when they receive light from some luminous source
and reflect it to our eyes.
Whether a body is luminous or nonluminous depends as much on its condition
as on the material of which it is made. By changing the conditions we
can make many familiar substances luminous or nonluminous at will.
The filament, or fine wire, inside an electric light bulb is nonluminous
unless it is heated by an electric current passing through it. We can
take a cold piece of iron and make it glow red, yellow, or white by heating
it in a bed of burning coals or over a gas flame. When solids and such
liquids as melted metals are heated to temperatures above 800 degrees Centigrate
(about 1500 degrees Fahrenheit), they become sources of light. Such
heated materials are known as incandescent bodies.
Careful observation shows that the light from candle flame comes from
many small, hot particles of carbon heated by the burning gases from
the candle wax. Thus the flame is another incandescent source of light.
Many of the carbon particles are not completely burned in an ordinary flame.
They cool off as they are carried above the flame by the air, become
nonluminous, and make up the main part of the rising smoke.
Not all light sources are incandescent. Neon tubes and fluorescent lamps,
like electric light bulbs, give off light as long as an electric current
passes through them. Touching each of them, however, convinces us at once
that there must be a difference in the way the light is produced. The neon
and fluorescent tubes remain quite cool, whereas the incandescent bulb
soon becomes too hot to touch. Pursuing this difference further, we find
that by gradually increasing the current in the filament of the incandescent
bulb we can increase its brightness, but that there is also an accompaying
change in color. At first we see a dull red glow which changes to a bright
yellow and, with sufficient current, can become "white hot" like the heated
iron. On the other hand, if we increase the current through a neon tube
to increase the brightness, we observe no change in color. Thus there
is a basic difference between incandescent sources and other sources.
In the former, changes in brightness, temperature, and color seem to be
closely linked, while in the latter the color of the source depends mainly
on the nature of the material and does not vary with brightness.
A great deal of light reaches our eyes from nonlumionus surfaces. To
convince ourselves, we need only imagine how the average room would
appear if the walls and other surfaces were covered with a paint so
black that it reflected none of the light reaching it. The lights would
appear as bright glares against dark backgrounds. White ceilings or
bright walls reflect and diffuse much of the light they receive and so increase
the brightness within the room. In fact, when we use indirect lighting
we hide the lamps from sight, and all of the light reaches us after
being reflected from the walls and ceiling. On a larger scale the moon,
which we often think of as a source of light at night, is really an
indirect-lightning device that reflects sunlight.
The moon is an indirect-lightning device that reflects sunlight.
Burned gases in flame become source of light.
