# Calculating the frequency of a photon

Gold Member

## Main Question or Discussion Point

Hi,
I've read that the frequency of EMR is found by means of a spectroscope , which uses diffraction grating and that it is derived indirectly from its wavelength. Is that true?

Is there a way, an instrument that count directly the frequency of a photon or a y-ray? and up to what frquency? Is it possible to verify that the frquency of a γ-ray resulting from the annihilation of an electron is 1.2356 x 10^20 Hz (511.000 eV)?
Thanks for your help!

## Answers and Replies

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Drakkith
Staff Emeritus
Science Advisor
Once we get into the IR range, we lose the ability to directly measure the frequency of the EM wave. Current electronics simply can't operate fast enough. Instead, we can measure the energy of the interaction, separate the incoming light into a spectrum, and a few other things to get the frequency/wavelength indirectly.

Edit: I feel I should also mention that it is impossible to directly measure the frequency of a single photon. A photon is a single event, whereas we need multiple photons to interact with our measuring devices to be able to track the frequency of the EM wave. If we observe a radio wave over time we are seeing millions and millions of photons per second, each one carrying a small piece of information about the wave.

Bobbywhy
Gold Member
Gold Member
Once we get into the IR range, we lose the ability to directly measure the frequency of the EM wave.
Edit: I feel I should also mention that it is impossible to directly measure the frequency of a single photon.
Thanks, Drakkith, but are you able to measure the oscillations in presence of a coherent laser beam of visible light? And how do you measure wave length with precision?

So, at LHC they measure the energy of the y-ray from e-/e+ annihilation only measuring its wavelength? how precise can it be?

Drakkith
Staff Emeritus
Science Advisor
Thanks, Drakkith, but are you able to measure the oscillations in presence of a coherent laser beam of visible light?
No, visible light oscillates at too high a frequency for electronics to follow.

And how do you measure wave length with precision?
A spectrograph can do this by separating the different wavelengths.

So, at LHC they measure the energy of the y-ray from e-/e+ annihilation only measuring its wavelength? how precise can it be?
They don't measure its wavelength. The wavelength is inferred based on how much energy the gamma ray gives to the detectors upon absorption. At least that's how I think it works. I'm not up to speed on high energy particle physics.