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We now all “know” that visible light has a wavelength between about 380 nanometres and 700 nanometres, and that shorter wavelength are the domain of UV, X-rays, and gamma rays; and longer wavelengths for infrared, microwave, and radio.

My question—and I have Googled far and wide and read quite few books to find the answer, but I guess I’m asking the wrong question—is: “How did we figure out the wavelengths of specific rays?”

I mean, for example, how did we figure out that yellow has a wavelength of 580 nm? (or any other color/band has any other wavelength)

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    $\begingroup$ You might have to read the history of interferometry. A. Michelson's was the pioneer of highly accurate wavelength measurements (Nobel Prize winner). $\endgroup$ – M. Farooq Nov 26 '20 at 3:51
  • $\begingroup$ Thanks @M.Farooq; I have found this video youtube.com/watch?v=j-u3IEgcTiQ which, while it may lack in wow! factor, helped me understand much better! $\endgroup$ – Pierre Paquette Nov 26 '20 at 4:14
  • $\begingroup$ Yes, it is an amazing piece of equipment and very expensive too. If you read early works on interferometry, it was so sensitive that if someone was walking on the floor, it would ruin the experiment. $\endgroup$ – M. Farooq Nov 26 '20 at 4:16
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    $\begingroup$ Here is the top hit when googling "measuring wavelength of light history" Thomas Young and the Wave Nature of Light:"Young used very simple equipment to produce patterns of light and dark bands: a candle and a card with a rectangular hole across which he stretched a single human hair. He used his observations to measure the wavelength of light." Details of the calculation follow. $\endgroup$ – Conifold Nov 26 '20 at 13:42
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It should not take googling far and wide, a standard college book on physics will give you the details.

Basically, Snells law on refraction, tells us that light is a wave.

Then as diffraction occurs when diffraction gratings are around the wavelength of the incident light, experiments with known diffraction gratings then tells us the wavelength of specific light - say that of purple light.

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    $\begingroup$ Snells law is equally compatible with a corpuscular theory, it is only diffraction and interference that suggest a wave theory. $\endgroup$ – Conifold Nov 26 '20 at 13:45
  • $\begingroup$ As I said, I was probably asking the wrong question. Thanks everyone! :) $\endgroup$ – Pierre Paquette Nov 26 '20 at 23:45

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