What is light?
Now what exactly is light? A Scottish physicist named James Clerk Maxwell (1831 - 1879) showed that electric and magnetic fields fluctuating together can form a propagating wave, which was named an electromagnetic wave. Light is this type of wave. Maxwell knew that a changing electric field produced a magnetic field. He hypothesized that a changing magnetic field would produce an electric field because most things in nature are balanced. Based on the work of Farady and Hertz, this was discovered this to be true. A common way to generate an electromagnetic wave is to have an antenna connected to an alternating current which will cause a changing magnetic field which will cause a changing electric field which will cause another changing magnetic field and so on. These waves are also produced in stars in the form of UV rays, X rays and Gamma rays.
An electromagnetic wave, like other waves, has a frequency f, a speed
v and a wavelength 
,
which are related by the equation 
.
In vacuum or in air, to a good approximation, the speed v = c (speed of
light: 
m/s) so the relationship would be 
Above shows the electromagnetic spectrum, which depicts electromagnetic waves over a huge range, for values less than 104 Hz to those greater than 1022 Hz. Because all of these waves travel at the same speed, this drawing can also be used to figure out the wavelengths. We can also see the grouping of the spectrum into radio waves, infrared, ultraviolet, x and gamma rays. Shown here the boundaries are sharp but in actual practice the boundaries are not so well defined and the regions overlap.
Beginning on the left are the radio waves. The lower frequency radio waves are generally produced by electric oscillator circuits while the higher frequency waves (microwaves) are generated using electron tubes called klystrons. The next set, called infrared or loosely referred to as heat waves, generally originate with the vibration and rotation of molecules within a material. The next set,which we are most familiar with, is the visible light waves, represented by a white stripe. These are generally emitted by hot objects, such as the sun or the filament of an incandescent light bulb where the temperature is high enough to excite the electrons within an atom. Ultraviolet rays are generally produced from the discharge of an electric arc and x-rays are generated by the sudden deceleration of high-speed electrons. And last, gamma rays are radiation from nuclear decay.
Of all the frequencies, the most familiar set, that of light, is the
smallest range indicated in the spectrum. The human eye can perceive only
the frequencies between 
Hz and 
Hz as visible light. For the most part, visible light is discussed in
terms of wavelengths with the unit being nanometers (nm) where 1 nm =
m
rather than in frequencies. Our brain recognizes these wavelengths as
different colors where the 750 nm is approximately the longest wavelength
of red light and 380 nm is approximately the shortest wavelength of violet
light. In between these two wavelengths are all of the other visible colors.
Wavelength also plays a very important part on other parts of the magnetic spectrum besides defining color for visible light. One important aspect about wavelength is that it determines how much a wave diffracts, or bends around an obstacle. Longer wavelengths bend around an obstacle more than shorter wavelengths. An example of why this is important is with radio waves. AM waves are significantly longer than FM waves and therefore they have a greater ability to bend around buildings than FM waves do.The inability of FM waves to diffract around obstacles is one of the major reasons why FM stations broadcast their signals from high areas in a "line-of-sight" manner.
