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
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
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.
Information supplied by: http://csep10.phys.utk.edu