We will focus on sound waves in describing the Doppler effect, but it works for other waves too.

Consider a stationary source of sound broadcasting a single frequency sound wave. The usual relationship between frequency, speed, and wavelength is:

f

=

v l

v represents the speed of sound through the medium.

Let's say you, the observer, now move toward the source with speed u_o. You encounter more waves per unit time than you did before.

What has changed, in effect, for the waves?

1. their speed
2. their wavelength

Relative to you, the waves travel at a higher speed:

v ^' = v + u_o

If you moved away from the source the relative speed between you and the waves would be lower. In either case, this shifts the frequency of the waves you hear, to:

f '

=

v +/- uo l

= (v +/- uo) *

f v

= f (

v +/- uo v

)

Use the first sign (+) when the observer moves toward the source and the second sign (-) when the observer moves away.