Light is a form of radiation. Specifically, electromagnetic radiation, which is another way energy propagating through space. Electromagnetic radiation comes in indivisible, discrete chunks called photons. The light that we see is a stream of these photons of a specific energy range hitting our cornea. The energy range that we can detect is limited by intrinsic physical properties of our eyes. In Astronomy, the telescope's job is to collect as many photons as possible.

So photons are chunks of energy. They have momentum and can be absorbed by other objects such as electrons. They are little particles, like bullets fired out of a gun. In fact, Albert Eienstein earned his nobel prize for proving this idea. But if you examine photons more closely, it turns out that they also behave like waves. This is called the wave/particle duality of light. The classic saying is that photons are both waves AND particles. But it's more appropriate to say that photons are neither waves NOR particles. Photons behave like waves if you test them for wave-like properties and they behave like particles if you test them for particle-like properties. They don't do both at the same time.

An electromagnetic wave derives its name from have two oscillating wave compononents: an electric field and a magnetic field.

Two cycles of an electromagnetic wave. Notice how the electric field oscillates at a right angle to the magnetic field.

All electromagnetic waves move at the aptly-named speed of light, denoted by the letter c. In a vacuum the speed of light is:

c = 299,792,458 m/s.

Most people simply round this to c = 3.0 x 10⁸ m/s. In non-vacuums, such as air, glass, water, etc., light slows down, but it's still pretty fast.

Since c is (essentially) a constant and is the equivalent of the wave speed, there is a very nice formula to interchange frequency and wavelength of electromagnetic waves:

where λ is wavelength in meters, c is the speed of light in m/s, and f is frequency in Hz (1/seconds). The local NPR station in Monterey, CA, KAZU, has an FM signal at 90.3 MHz (mega-Hertz, or millions of Hertz). To calculate the wavelength of the KAZU signal:

λ = (3.0 x 10⁸ m/s)/(90.3 x 10⁶ s^-1),

λ = 3.3 m.

The signal sent from from KAZU 90.3 FM is a little over 3.3 meters long, or about 11 feet. It oscillates 90,300,000 times a second.

The light we see (visible light) is electromagnetic radiation over a specific range of wavelengths, between 400 nm and 700 nm (400 x 10^-9 meters and 700 x 10^-9 meters). We see different wavelengths as different colors from Red to Violet (ROY G BIV). Reds have the longest wavelengths we can see and the violets have the shortest. All the other colors fall inbetween.

You may be wondering what happens when we extend the wavelength of electromagnetic (EM) radiation beyond our visual range. Well wonder no more! The total range of all possible wavelengths is called the electromagnetic spectrum. Different ranges of EM radiation are given names, some of which will be familiar. Here is a picture of the spectrum:

If you start at violet (~400 nm) and start making the wavelength smaller, you move into the ultraviolet range of the spectrum. Beyond that is x-rays, as used at the dentist's or at the hospital. Gamma rays are extremely high-energy and very dangerous but not encountered often, as the Earth's atmosphere absorbs this radiation.

If you start at red (~700 nm) and start increasing wavelength, you'll move into the infrared part of the spectrum. Infrared radiation is most commonly sensed as heat. Beyond infrared are microwaves (as used in microwave ovens), then radio (FM first, then AM), and extending out to shortwave. EM waves get rather large; AM radio, for example has wavelengths on the order of 1 kilometer.

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