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Name: Brice F.
Status: student
Age: 9
Location: N/A
Country: N/A
Date: 1/14/2004


Question:
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?


Replies:
Brice,

"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.

Regards,
ProfHoff 778


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.

Vince Calder


Brice,

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
Physics Professor
Illinois Central College



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