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Name:  Craig
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What is color and how does it differ from color temperature?

Dear Craig,

The color of a given light source is determined by the frequency or the light or, equivalently, by its wavelength. The frequency is the number of oscillations per second and the wavelength is the distance light travels in one complete oscillation. Visible light varies between red light, with a wavelength of 700 nm and a frequency of 430 trillion Hz and violet with a wavelength of 400 nm and a frequency of 750 trillion Hz. A nm is a billionth of a meter and a Hz is 1 oscillation per second. If the light includes several frequencies, the eye will interpret the light as having several colors, but usually combine them as, for example yellow and blue are seen as green. All colors combined are seen as white.

Color temperature is more complicated. If you have a black body at a certain temperature, that body will radiate a continuous spectrum whose average frequency increases with the temperature. Since red is the lowest frequency light, when you heat something up, it will first glow red. When it is hot enough so it emits all frequencies roughly equally, it glows white hot. So the temperature of a glowing body, such as an incandescent bulb can be determined by its color. That temperature is then called the color temperature since it determines the color of the light emitted by the bulb.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University

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