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Review of Waves and Ray Diagrams

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.

<img SRC="period.jpg" BORDER=0 height=300 width=400>

The frequency f 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".

<img SRC="16a.jpg" BORDER=0 height=384 width=512>

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.


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Copyright © 1999 Rensselaer Polytechnic Institute and DJ Wagner. All Rights Reserved.