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What is Doppler Effect in Light?

Doppler effect is a particular type of phenomenon where the frequency and wavelength of wave energy (like sound or radio waves) between two objects seem to change with the shift in the objects’ position. Unlike sound and radio waves, light does not require a medium of travel or propagation. Therefore, from Einstein’s relativity theory, differentiating between whether the light source or the onlooker is moving is not easy. Thus, we can only determine the doppler effect in light by observing the relative movements of the observer and the light source.

Doppler effect explained

The two’s relative motion determines variations in wavelength and frequency of wave energy between a wave source and an object. The closer a siren-sounding car gets to an onlooker, the higher the pitch of the siren. The decrease in distance between them leads to compaction of sound waves that increase the frequency and decreases wavelength, thus the high pitch. The opposite is true with relative motion away from each other.

Doppler effect in the light

Ideas derived from careful study of one wave can be applied in another wave type. That is true for both light and sound waves. The Doppler effect’s relevance in sound waves is only limited to when the sound-producing object is moving. As mentioned above, light requires no medium to be propagated. The variation in wavelength and frequency of light waves is determined by the direction of movement of the light source and the onlooker, considering Einstein’s theory of relativity.

Since we learned from relativity theory that there is no way of figuring out whether it’s the onlooker or the source of light moving, the Doppler effect’s determination can only be aided by two equations. One equation illustrates the movement of the light source and the onlooker towards each other and the second one depicts the movement of the two farther apart.

Generally, when movement is towards each other, a decrease in wavelength and an increase in light waves’ frequency is noted. On the other hand, movement apart leads to increased wavelength and decreased frequency of light waves. Variations between observed and emitted light frequencies as the light source moves or onlooker move relative to one another give rise to the light’s Doppler effect.

The shift in light frequency in an electromagnetic spectrum leads to color changes observed with the naked eye.

Redshifts and blueshifts

A careful study of the color spectrum implies that lights of different colors have varying wavelengths and frequencies. Therefore, when the wavelength shifts, the frequency follows suit. A shift that leads to a longer wavelength will give rise to a lower frequency. On the other hand, a shift involving a shorter wavelength results in a higher frequency.

A light source moving towards the observer results in light waves shifting to shorter wavelengths, a phenomenon referred to as “blueshifting”. On the contrary, a light source moves away from an observer leads to light waves shifting to longer wavelengths, and a phenomenon referred to as “redshifting”.

The bottom line

In a nutshell, the Doppler effect is simply a phenomenon where a wave source’s relative motion and an observer lead to variations in wavelength and frequency of wave energy (sound, radio, or light waves). Whereas sound and radio waves require a medium of propagation, light doesn’t. Thus, the Doppler effect comes to action only when the light source and the observer shift their positions relative to each other.

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