When discussing the Doppler Effect in sound, we have relationships for the obser

Question

When discussing the Doppler Effect in sound, we have relationships for the observed frequency if the source was moving and if the observer was moving .

Fo = Fs (1 +- Vo/Vwave)

These same equations work for electromagnetic waves. They can also be simplified if the relative velocity between the source and the observer (vrel) is much less than the speed of the waves. The speed of the electromagnetic waves is the speed of light, c = 3.00Ý108 m/s. The formulas can be simplified to:

Fo = Fs (1 +- Vrel/c) if Vrel << c

where fs is the frequency of the source and fo is the observed frequency.

Suppose a space probe emits a radio signal at a frequency of 3.900000 MHz (Megahertz). It travels away from Earth with a speed of 2.200Ý104 m/s. What is the frequency of the signal when it is detected on Earth? (Note: give 7 significant digits)

In many situations, the difference in frequencies is not very large, and we often are only interested in the difference in frequencies, f = fo-fs. The equation above can be rewritten as:

delta F = +- FS Vrel/c

Suppose a radar gun being used by a police officer is operating at a frequency of 7.120Ý109 Hz. You are traveling towards the parked police car at a speed of 22.90 m/s. What is the difference, f, between the frequency detected by your radar detector and the frequency of the source?

Explanation / Answer

Think of an ambulance with its siren on, coming down the street towards you.
As it passes you, the pitch of the siren is lower than it was before it passed you.
That is an example of the Doppler effect.

When light waves or sound waves are emitted by an object, they radiate out in all directions. If the source is moving fast enough (in relation to the wavelengths), then the waves will be bunched up in front of the object and spread out behind it.
The bunched up waves will be at higher frequencies (in sound, the higher frequencies are higher pitched to our ears, in light the higher frequencies are towards the blue end of the visible light spectrum).
Christian Doppler formalized the concept in 1842 (the idea was known to the ancients).

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