I was wondering how we determine the wavelengths of light. I know that we could probably use a spectrometer to compare the wavelengths to a given set, but I was wondering where the given set comes from. In other words, did someone at some point in the past, perhaps Newton, run white light through a prism and then perhaps determine the individual wavelengths for the colors by the distance they fall from the base of the prism maybe? If so, wouldn't this make the wavelengths of light arbitrary? In other words, are the wavelengths absolute or are they just ratios of the given definition for a particular color? Since we can also measure the photon count of light waves, does this mean we can actually pick out the separate pulses from each individual "ray", or is this a statistical thing?

If the measure of the wavelengths of light are due to an initial given wavelength, or the position that they fall for a particularly angled prism, then that would make the frequencies related in this way as well. They would simply be determined by f=c/w and otherwise have no real value except for a ratio of energies. This ratio is also found by a constant of proportionality known as Planck's constant, h. It relates the energy of light to its frequency. But many calculations that include h are found to then be confounded by another proportion, which is the alpha constant, 1/137.0359896. It has no units, just a pure number for proportionality. If the true value of h were 137 times smaller, it might rid us of some of this. The actual frequencies of light, however, would have to be 137 larger than what we measure, to provide the same energies. The alpha constant would still exist, but only as the ratio of the average velocity of the electron in hydrogen to the speed of light. It would then be incorporated only into equations involving electrons.

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"Let's define another operator, Sz, which we won't pay any attention to."
"This transformation will automatically make zero equal zero."
"It may be true that zero equals zero -- and that is certainly an equality -- but I don't want to go into the details at this time."

They're absolute.
Passing the light through a diffraction grating and measuring the angles of the resulting interference pattern would be one way to measure the wavelengths directly, though I don't know if that was the approach first taken.

Grant Hutchison

In high school physics we measured the wavelengths of light in several ways. One was to have two glass surfaces very close together, and we looked at the patterns as the bouncing light interfered with itself. Another was with the two slit interference patterns.

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