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All About Spectra

Visible Light

Visible light is made up of electromagnetic waves, vibrations of electric and magnetic fields that propagate through space. In contrast to slow-moving ocean waves to which they are analagous, electromagnetic waves travel at the speed of light: 300 million meters per second, or 669.6 million miles per hour!

Every electromagnetic wave exhibits a unique frequency, and wavelength associated with that frequency. For instance, this picture represents an electromagnetic wave corresponding to the color red.

Its frequency is 428 570 GHz (pronounced gigahertz), which can also be stated as 428,570 billion cycles per second. So when you look at red light, your eye receives over four hundred trillion waves every second!

The wavelength of such light is but 700 nanometers long ,which means that one wave spans only 7/10 000 000, or 7 ten millionths of a meter. Measuring a single wavelength of this light against the length of a meter is like comparing a thumbtack's diameter to the distance across the United States.

All electromagnetic waves are classified according to their characteristic frequencies, into what is known as...

The Electromagnetic Spectrum

JPEG (40K)

Just as red light has its own distinct frequency and wavelength, so do all the other colors. Orange, yellow, green, and blue each exhibit unique frequencies and consequently wavelengths. While we can perceive these electromagnetic waves in their corresponding colors, we cannot see the rest of the electromagnetic spectrum.

Most of the electromagnetic spectrum is invisible, and exhibits frequencies that traverse its entire breadth. Exhibiting the highest frequencies are gamma rays, x-rays and ultraviolet light. Infrared radiation, microwaves, and radio waves occupy the lower frequencies of the spectrum. Visible light falls within a very narrow range in between.

So radio waves, such as those that FM radio stations broadcast, are simply electromagnetic waves with a much lower frequency and longer wavelength than visible light. The millimeter radio waves, also called microwaves, possess a slightly higher frequency and shorter wavelength than the FM radio waves.

The Cosmic Barcode

Notice the three highlighted regions in the picture of the electromagnetic spectrum above. The leftmost region shows which frequencies the typical FM broadcast stations use. The middle highlighted region indicates which frequencies the BIMA array detects.

The electromagnetic spectrum represents a continuum of frequencies. However, radiation emitted by stars, galaxies, and other distant objects falls into more discrete frequencies. In combination, these frequencies serve as "cosmic barcodes"; they can reveal a lot of information about the composition, structure, and motion.

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NCSA. Last modified 11/10/95