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The wavelength or frequency of monochromatic electromagnetic radiation is determined indirectly by calculations on the interference or diffraction pattern. These patterns are generated by gratings, crystals or also the beam splitting at semitransparent mirrors. Are there direct measurements of the wavelength or frequency of EM radiation beyond radio waves?

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  • $\begingroup$ You can buy a tabletop demonstrator/experimentation kit in the microwave band from several suppliers of physical teaching equipment. They can be configured in modes where you examine both grating interference and Michelson interferometry. $\endgroup$ Sep 27, 2019 at 21:50
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    $\begingroup$ Please ask only a single question per thread - you know how the site works. I have edited out an unrelated question which should be asked separately. $\endgroup$ Sep 30, 2019 at 23:18

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Are there direct measurements of the wavelength or frequency of EM radiation beyond radio waves?

For frequency it depends on what you are willing to call "direct", but there is a reasonable (though not necessarily bulletproof) case to be made that frequency measurements using frequency combs for the bill. (In short, you set up and lock the frequency comb, with a comb separation that can be measured directly in the radio regime, and then you count up the comb teeth up to the line you want to measure; you then also measure the separation to the nearest comb tooth by measuring the frequency of the beating between the tooth.) I don't think any semantic debate about whether this measurement is "direct" is at all interesting, though.

For wavelength, the answer is an unambiguous yes; the attosecond-streaking method I described in my answer to Have we directly observed the electric component to EM waves? is unambiguously a direct length measurement.

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The first part of the question, "Are there direct measurements of the wavelength or frequency of EM radiation beyond radio waves?", needs a definition of "direct". If you mean something like counting wave peaks as they pass by, then dividing by the time over which we are counting, the answer is "no". But it is possible to do something that does directly measure frequency, at least as directly as using a ruler to measure the length of a stick.

We have frequency references that are extremely stable, such as a CO2 laser frequency-locked to an absorption peak in methane. Another laser's frequency can be compared to that reference frequency by illuminating a fast photodetector with both lasers and counting peaks of the detector's output. The frequency of the second laser is just the sum of the reference frequency and the frequency of the detector's output.

This is a temporal interference based measurement, but length measurement using a vernier scale is very, very much like an interference measurement and can be called a direct measurement.

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Yes there is. In nuclear physics, for example, Germanium or silicon crystals are used in detectors for measuring the energy of gamma rays. The $Energy = h \nu$. There are other detectors used for x-rays as well.

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