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Is there an instrument that measures the frequency of light DIRECTLY? By "DIRECTLY" I mean without using any properties of the Wavelength.

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    $\begingroup$ What sort of light? I believe you tune your radio to a frequency. $\endgroup$ – Rob Jeffries Feb 15 '16 at 12:45
  • $\begingroup$ And caesium clocks count the individual cycles in a microwave $\endgroup$ – John Rennie Feb 15 '16 at 12:48
  • $\begingroup$ Above microwave: There are no direct ways of measuring frequency, because there are no recording devices capable of operating anywhere near optical frequencies. $\endgroup$ – Anubhav Goel Feb 15 '16 at 12:50
  • $\begingroup$ @RobJeffries and John Rennie posting answers as comments? These suddenly give context to the question. $\endgroup$ – Bill N Feb 15 '16 at 17:21
  • $\begingroup$ en.wikipedia.org/wiki/Frequency_comb $\endgroup$ – Norbert Schuch Feb 16 '16 at 0:02
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The photoelectric effect can be used to derive the frequency of the photon directly. This is because only a certain threshold frequency will allow for the emission of electrons from a metal. Therefore judging from the energy of the emitted electrons, the frequency can be observed.

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  • $\begingroup$ Does this mean there IS an instrument with which the frequency of starlight can be measured directly? $\endgroup$ – Phil Feb 17 '16 at 21:17
  • $\begingroup$ It depends on what you consider directly. Really this method measures the photon energy which is related by plancks constant to the frequency of the light. $\endgroup$ – Jaywalker Feb 18 '16 at 7:40
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You have to be clear as what you mean by "frequency" and "directly." If what you mean is an instrument like an oscilloscope with a photo-detector, where the light is incident on the photo-detector without any other source coincident with it. Then what you'll need is an oscilloscope and a photo-detector capable of responding to electric fields at $10^{15}$ Hz, and as such does not yet exist.

Since most photo-detectors are square-law detectors, ie. they respond not to the electric field but to the modulus squared of the field (also known as intensity), then the information is lost. However, if you performed what's known as a heterodyne or homodyne detection method whereby a reference light field is added to the light field you'd like to measure then you can measure the beat frequency of the source, provided the beat frequency is within the bandwidth of the apparatus.

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  • $\begingroup$ The first paragraph seems to be confirming that direct measurement of frequency is not possible. The second paragraph seems to be using the wavelength to accertain the frequency. $\endgroup$ – Phil Feb 17 '16 at 21:15
  • $\begingroup$ Wavelength and frequency are a measure of the same thing $\endgroup$ – JQK Feb 17 '16 at 22:35

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