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# Quantum Physics - Sixth Form watch

1. Here's the question:

I'm facing problem with (c)(ii). My concept tells me that rate at which photons travel depends on the Intensity of the radiation and max. kinetic energy of released electrons depends on the frequency of the radiation - so answer to this part should be that max. kinetic energy of the electrons increases(as wavelength decreases, frequency increases; increasing energy of a photon), but n, number of electrons ejected per unit time remains constant. But mark scheme says that n decreases:

Examiner report suggests an explanation for this, but I really don't understand it:

What is the relationship between Intensity, energy, and the rate of arrival of photons? Doesn't constant Intensity mean that the rate of arrival of photons is constant? Any helpful explanation would be appreciated. . .

-Thanks
2. The (radiant) intensity of a source of light (photons) is a measure of the amount of energy arriving per second at some point. (Usually a defined area).

So in the case of the photons in this question, that would be made up of 2 parts
*the number of photons arriving per second and
*the amount of energy each one carries.

You need to be careful whan dealing with the photoelectric effect, especially when talking about the effect of increasing the intensity of the light incident on the metal. It's always qualified by saying that, if the radiation is of a single (constant) frequency, then increasing the intensity of the beam will increase the number of emitted photoelectrons. This is because, in this case, you need to increase the number of photons striking the surface per second to increase the amount of energy per second.
Increasing the frequency of the photons will also increase the amount of energy striking the surface per second. (Even if the number per second remains the same)
So the point being made is that if you increase the frequency (lower wavelength) of the photons but keep the intensity constant, you are keeping the rate of arrival of the radiant energy constant. However, if each photon has more energy (higher f) then to keep the rate of energy arrival constant, you would need to reduce the number of photons arriving per second. This in turn will reduce the number of photoelectrons emitted.
3. Wow, massive thanks for such an awesome explanation. (PRSOM)

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Updated: December 1, 2011
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