Unlike the oscillations of sound waves, the oscillations of light are so fast that extremely complex equipment is needed to observe them directly. However, it is possible to measure the frequencies of these oscillations indirectly with frequency combs. These combs are made up of a set of regularly spaced ‘teeth’ where each tooth corresponds to a frequency. Used as a graduated ruler, they offer the possibility of measuring an optical frequency with great precision. This makes it possible, among other things, to measure variations in the distance between the Earth and the Moon with an accuracy equivalent to the size of a hair.
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Source: Phys.org