Doppler effect is related to motion-related frequency changes of waves, and was proposed in 1842 by Australian physicist Johann Christian Doppler. Although first proposed and developed for sound waves, this effect works can also be applied for light. The Doppler effect is based on the principle that the frequency of a wave is different when there is a relative motion between the source and the receiver, than it would have been had there been none.
The Doppler effect equations for a moving detector but a stationary source, and a moving source but a stationary detector are different.
For the first case,
if the detector is moving towards the stationary source with a velocity of vd, the new frequency for the emitted wave will be
f' = f (1 + vd/v), where v is the velocity of the wave.
However, if the detector is moving away from the stationary source with the same velocity, the new frequency will be given by
f' = f (1 - vd/v), where v is the velocity of the wave.
for the second case, if the source is moving towards the stationary detector with a velocity of Vd, the new frequency for the emitted wave will be
f' = f ( v/(v + vd) ), where v is the velocity of the wave.
However, if the source is moving away from the stationary detector with the same velocity, the new frequency wil be given by
f' = f ( v/(v - vd) ), where v is the velocity of the wave.
There is however, a difference in the application of Doppler effect for sound and light waves. There are two reasons for this. First, that the sound waves are mechanical and require a medium to travel. But, light waves are electromagnetic and do not require a medium to travel through. Second reason is that whereas the velocity of light waves is the same in all inertial frames, the velocity of sound is not.
Even though the Doppler effect equations for light and sound are completely different, at low speeds they both produce approximately the same result. Hence at low relative velocities between the source and the detector, the same equation can be applied for light as well.
Since measuring of the wavelength of light in astronomy is easier than measuring the frequency, the Doppler effect equation can be changed to
Δλ = uλ/c
Δλ is the magnitude of Doppler shift, c is the velocity of light, and u is the relative velocity between the source and the detector such that u<
The Doppler effect has a major application in the field of space research and has helped scientists towards a major discovery - that the galaxies were moving away, from which the scientists have realized that the universe is expanding. It has also been found that the distant a galaxy, the faster it is moving away.