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We first introduce some terminology
associated with waves. Let us imagine that we drop a rock in the
middle
of a still pond and watch the waves emanating out from the center.
From above the wave crests might appear as in
Fig. 22.2.

We assume the wave pattern is regular, and consider the following characteristics of these waves.

**Definitions**

- The
**wavelength**( ) is the distance between neighbouring crests or troughs. - The
**speed**(*v*) is the rate at which the crests (or troughs) move forward. - The
**Period**(*T*) is the time that elapses between passing crests (or troughs). The period can be expressed in terms of the speed and wavelength:*T*= . - The
**frequency**(*f*) is the number of crests (or troughs) that pass by per unit time. It is equal to the inverse of the period:*f*= .*T*above we get the useful expression:*v*=*f*. - A
**ray**is a line drawn from one wave crest to another which intersects each crest at right angles, as in Fig. 22.3. For light waves, the rays always point in the direction of the motion. Rays therefore provide a useful representation for describing the motion of light waves.

10/9/1997