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up vote 7 down vote favorite

My question is pretty brief. When two sound waves of nearly same frequencies interfere, we get beats.

But, I have not observed something like that happening in the case of light. In fact, most of the light around us is a collection of continuous wavelength range which must be all nearly same frequencies.

Can we observe beats in light waves, as in case of sound waves?

If yes, how to observe them?

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2 Answers

up vote 9 down vote accepted

The math of beats is absolutely the same for light as it is for sound, but ...

  • Most light around us in incoherent, so does not form beats on macroscopic time or distance scales.

  • It is hard to select to sources that could conceivable give visible beats Look at it this way the frequency of green light is around $ c /(500\text{ nm}) \approx 6 \times 10^{15} \text{ Hz} $. All the colors have frequencies of the same order of magnitude, so it is very hard to the beat frequencies of two randomly selected colors tend to be around $10^{13}$--$10^{14} \text{ Hz}$: much faster than your eye can detect. To construct a pair of sources that you could see beat would require controlling their frequencies to about 1 part in $10^{14}$. That's not easy.

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+1. It might also be worth adding that the interference fringes observed in the double slit experiment are very closely related to the phenomenon of beats. – Nathaniel Mar 7 at 1:23
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+1 The key point is that most of the light around us is from incoherent sources. The easiest way to obtain beats is to start from a single source (a laser for instance), split it in two, shift the frequency of one the beams (using an acousto-optic modulator for instance) and have them interfere again. The signal generated by a photodiode will exhib a beat. – Oli Mar 7 at 3:36
@Oli Something similar was done by my Physics teacher. Those things looked like beats, but were not formed using the same procedure as for sound waves. That is why I preferred to take the forum's opinion. – Cheeku Mar 7 at 5:01
@Cheeku I'm not sure what you mean by "not the same procedure". It is the same physical effect, it is just that it is difficult (but not impossible) to make two independent sources interfere. – Oli Mar 7 at 6:26
Interference fringes, of course, arise from the same math in the spatial domain. Even there we use finely manufactured physically small features (or differences in feature length scales) to magnify their size to something we can see with the mark one eyeball. – dmckee♦ Mar 7 at 17:27
up vote 5 down vote

A laser gyroscope detects 'beats' of light.

In a laser gyro, laser light goes in two directions 'round a ring, clockwise and counterclockwise. If the gyro is rotating, one laser direction will be at a slightly different frequency than the other, and their interference will create a 'beat'.

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