Color and Temperature
Name: Brice F.
Are the colors of the visible light spectrum in the EM (electro-magnetic)
spectrum different temperatures?
Are the color temperatures going to always read close to the temperature of the room?
If the colors are different temperatures,which are the hottest and which are the coldest?
Will the temperatures coldest to hottest go from violet to red?
"Heat" is a measure of a system's energy content. "Temperature" is the localized measure
of energy wherever the thermometer is located. Indeed, if objects are heated enough their
measured temperature will rise. Still, there is a big difference between "heat" and
"temperature." No matter where you put a thermometer in a kettle of boiling water, the
temperature is the same. This is also true if you were to measure the temperature in
different parts of a teacup full of boiling water. Clearly, there is a lot more heat
(energy) present in the kettle of water. So, as you can see, temperature does not measure
an object's energy content.
"Temperature" is not related to the color of a room temperature object. In other words,
the color of an object like a piece of red paper or a blue shirt is not related to its
temperature. Confusion arises when one considers the way photographers or artists may
refer to the color of light. To a photographer, reds are described as "warm" colors and
blues are "cool."
Colors of electromagnetic radiation in the visible part of the spectrum do represent
different energies, but not different temperatures. In actuality, blue light is more
energetic than red light because blue light is of a shorter wavelength than red. From
that standpoint, the energy of visible light wavelengths range (from lower to higher)
red, orange yellow, green, blue, indigo, and violet. If you had a sample of materials
representing all those colors, all samples would have the same temperature as the place
where they were located.
Try it and see for yourself. Collect a bunch of different colored pieces of cloth. Put
all of them in a closed cardboard box and allow it come to room temperature. Poke a
thermometer through a hole in the box and measure the temperature of the different
pieces -- you'll discover that they will all be the same temperature. Thus, you have
proven to yourself that the temperature of the pieces of cloth has nothing to do with
their different colors
The "heat" part of the electromagnetic spectrum is represented by wavelengths that are
longer than any of the wavelengths that represent visible light.
There are three ways an object can appear to have a "color" in the every-day way in
which the term "color" is used. The first is when light, say from the Sun strikes the
object. Sunlight contains all the colors in the visible light in about the same amount
(We call that 'white light'.), so if the body absorbs lets say blue light the object
will appear yellow. That is, it is 'white light' MINUS the blue. If the body absorbs
red light, the body will appear green. Again, white light MINUS it red component. This
way of producing "color" does not depend upon the temperature of the object.
The other way an object can appear to have a "color" is if it is so hot it radiates
light. If the temperature is low enough, the object may not appear to have any color
at all, but may "feel" warm. This is because the object is radiating energy that the
eye cannot see, but the skin can feel as heat. As the temperature of the object
increases, the object will begin to "glow" a dull red. As the temperature increases
more the radiant color becomes increasingly orange, then yellow, then more blue,
until it becomes "white hot" to use the every-day word. The reason the Sun appears
white is because it is so hot. The faint yellow color that sunlight seems to have is
due to scattering of light by the atmosphere, but if you were to observe the Sun from
a spacecraft, or even a high mountain, it would appear more nearly white. This way of
producing "color" obviously depends upon the temperature of the object, and does so
in an understood, but rather complicated way.
The other way an object appears to have "color" is for it to absorb ultraviolet light
(black light as it is called) and to re-emit visible
light. This way of producing "color" is called "fluoresence". Many minerals fluoresce
strongly when they are exposed to ultraviolet light.
The colors of the visible light spectrum do not have temperatures. Blue light is harder
to make than red light. It takes more energy.
When something is heated enough, it can glow. At first, the color is red. As the
temperature goes up, the color goes from red to yellow, and then to white. This is
why "white hot" is much hotter than "red hot". If made hot enough, it will have enough
energy to glow blue. The hottest visible flames are violet.
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012