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Old Sep 11th 2009, 07:10 AM   #1
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Basic question on Photons

A question which we had in a test.

1)The power output of a source of light emitting photons of a fixed frequency is increased. How does the number of photons per unit volume in the beam change?
2)What will happen to the number of photons per unit volume if the frequency is increased keeping the power output constant?

The first part is pretty easy, the number of photons per unit volume will increase if the power output of the source increases keeping the frequency fixed

For the second part, I thought the number of photons per unit volume wouldn't change if the power output was kept constant while the frequency increases. But the energy of an individual photon would increase.

But it seems I'm wrong. As the power output is constant, and the energy of the individual photon increases, the number of photons per unit volume should decrease.
My stupid question:
I don't quite understand the meaning of the italics. I mean how can the power output of the source be kept constant while increasing the energy of the quantum? How is it achieved?
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Old Sep 11th 2009, 09:22 AM   #2
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Well all you need to do is increase the frequency of the photons but this also decreases the number of photons being ejected per unit time, requiring the machine to be on for longer to supply the same amount of power.

$\displaystyle P = \frac {W}{t}$
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