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Q & A: counting photons

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Most recent answer: 09/14/2012
Q:
hi How many photon are there in quantum?and how we can measure theorically them thanks
- siavash (age 25)
Ireland
A:
A photon is just the name for a single quantum of electromagnetic radiation, including light. So if I understand your first question, the answer is one.

For light, counting photons is straightforward. A carefully made photomultiplier tube (PMT), held at, say, dry ice temperature, will rarely give out an electronic blip unless it absorbs a photon. So you can look on an oscilloscope and count the blips to count the photons absorbed by the active part of the PMT. Usually this is not done by eye but rather by an automated electronic circuit. Certain diode detectors can be substituted for the PMT. This is all rather standard lab technique.

But you asked about measuring them "theoretically". That simply requires measuring the total energy in the beam and dividing by the energy per photon, hf, where h is Planck's constant and f is the light frequency. You can get the total energy in many ways, for example by seeing how much the beam heats up a bit of water with dye in it.

Mike W.

(published on 09/14/2012)

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