Dr. Aaron Titus | Department of Physics, High Point University
PHY1050      Astronomy of Stars, Galaxies, and the Cosmos
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part 2

White light is composed of all the visible colors.

A piece of glass can be used to separate white light into its component colors. You may have noticed the spectrum created by a crystal chandelier or the reflector from a bicycle. In the lab we typically use a grating (with slits etched in a piece of glass or on a mirror) that separates the light into its colors. A CD can act like a diffraction grating. You may have noticed that a CD separates reflected light into various colors.

Each color represents a range of wavelengths (or frequencies). The total range of wavelengths that our eyes can see is called visible light and is shown below. This is called the spectrum of visible light.

The colors of the visible spectrum are shown with lowest wavelength on the left to highest wavelength on the right. In order of increasing wavelength, the colors are violet, blue, green, yellow, orange, red.

You should know the colors of the visible spectrum in order from lower wavelength to higher wavelength. Just think ROY G BIV which you may have learned as a kid, except that we don't really consider I (indigo) as part of the spectrum. After all, we can all find the other colors just by looking at the spectrum. But can you really find indigo? I certainly can't.

The wavelength of visible light is very, very small, about a thousandth of a millimeter. Therefore, we use units of nanometers (nm) to describe the wavelength of light. The prefix nano means one billionth. The range of wavelengths for the various colors are listed below.

RegionWavelength (nm)Frequency (Hz)
Violet440 nm - 400 nm6.8 x 1014 - 7.5 x 1014
Blue480 nm - 440 nm6.3 x 1014 - 6.8 x 1014
Green560 nm - 480 nm5.4 x 1014 - 6.3 x 1014
Yellow590 nm - 560 nm5.1 x 1014 - 5.4 x 1014
Orange630 nm - 590 nm4.8 x 1014 - 5.1 x 1014
Red700 nm - 630 nm4.3 x 1014 - 4.8 x 1014

Visible light is not the only type of light. In fact, our eyes are tuned to a very small region of the entire electromagnetic spectrum. Astronomers, in fact, use all regions of the electromagnetic spectrum so that we can "see" radio waves, infrared waves, ultraviolet waves, x-rays, and gamma rays.

The entire electromagnetic spectrum is shown below, once again with lower wavelength on the left and higher wavelength on the right. Notice how narrow the visible region is. That's why astronomers like to "see" with all of the other wavelengths too!

You should know the regions of the electromagnetic spectrum in order from lower wavelength to higher wavelength. They are gamma rays, x-rays, ultraviolet (also called UV), visible, infrared (also called IR), and radio waves.






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