Wave Properties
of Light

In modern physics, light or electromagnetic radiation may be viewed in one of two complementary ways: as a wave in an abstract electromagnetic field, or as a stream of massless particles called photons. Although either is an acceptable description of light, for our purposes in introductory astronomy the wave description will be more useful.

Light as a Wave

The quantity that is "waving" is the electromagnetic field, an esoteric but quite measurable entity: your lights shine and your microwave runs and your radio plays because the electromagnetic field exists. As illustrated in the adjacent image, a wave has a wavelength associated with it.

It is normal to give the wavelength of light the symbol Greek "lambda". Some common units of length employed for wavelengths of electromagnetic radiation are summarized in the following table. (The micrometer is often still called by its old name, the micron.)

Units of Wavelength
Unit Symbol Length
centimeter cm 10-2 meters
Angstrom 10-8 centimeters
nanometer nm 10-9 meters
micrometer nm 10-6 meters

Wavelength, Energy, and Frequency

The speed of light in a vacuum is commonly given the symbol c. It is a universal constant that has the value
c = 3 x 1010 cm/second

The speed of light in a medium is generally less than this. Normally the term "speed of light", without further qualification, refers to the speed in a vacuum.

A wave can be characterized by its wavelength, but we can also characterize it by the frequency (how many wavelengths pass a fixed point in a given time; think of sitting on the dock---of the bay---counting the number of water waves passing in one minute) and the energy that it carries (think of a water wave knocking you over in heavy surf). For light waves the relationship among the wavelength (usually denoted by Greek "lambda"), the frequency (usually denoted by Greek "nu"), and the energy E are

where c is the speed of light and h is another universal constant called Planck's Constant that has the values
h = 4.135 x 10-15 eV-sec = 6.625 x 10-27 erg-sec

in two different useful sets of units (eV stands for "electron volts"; electron volts and ergs are two common units of energy). Thus, these equations allow us to freely interconvert among frequency, wavelength, and energy for electromagnetic waves: specifying one also specifies the others.

Some Light Exercise

Here are some exercises illustrating the conversion among energy, wavelength, and frequency for electromagnetic waves.

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