Light travels as a wave. You should have already encountered waves in your
physics course before starting this module. But in case your study of those
topics is a distant memory or scary nightmare, here is a review of some
of the key concepts.
Periodic waves traveling through space, such as light waves, are described
by their wavelength and period. A wave is periodic if it
repeats itself. Some examples of repeating and non-repeating waves are
shown here.

The wavelength
of a wave is the distance
between two repeated points, as shown in the figure above. The period T
of a wave is the time it takes for an entire cycle of the wave to pass
one point in space. In the animation below, one complete cycle passes
the green line in 5.0s, so the period of the wave is 5.0 s. (The
timer may or may not represent real time, depending on your system.)
Click on "replay" to see the animation again.

The frequencyf of a wave is the number of cycles completed
per unit time. In our animation, the wave travels through one fifth of
a cycle in one second, so the frequency would be 1/5.0s = 0.20Hz. Frequency
and period are just the inverse of each other:

f = 1/T.

The wave speed is the speed at which one point in the cycle travels
through space. Since any point on the wave will travel one wavelength of
distance in one period of time, the speed v can be found from

v =
/T =
f.

Representing Light

Light can be represented by sine waves, wave fronts,
or arrows in the direction of motion, as the animation below illustrates.
Click on "replay" to see a given animation again. When you have viewed
animation 1, click on the "2" in the bottom right to continue, then "3",
then "4".

A particular representation (waves, wavefronts, and rays) may be better
suited for a given context, but all three representations are equally valid
ways of illustrating light.