I am trying to programme a photoelectric effect simulator and the following problem has come up which I am stuck on.

Suppose I am given the power of a light source, The wavelength of light and the surface area of the metal, Can I calculate the number of photons incident on the metal surface? Additionally, Suppose I said the light source is operating at 40% intensity, how would intensity affect the number of photons incident on the metal surface?

Apologies in advance if I have not provided enough information or if anything I have said is incorrect.



1 Answer 1


You've provided enough information. Assuming every photon emitted is of the same wavelength, we can use the following equations. First, to related power and intensity:

$$I = \frac{P}{4πr²}$$

I for intensity, P for power, and r for distance between the light source and the metal target. If you wanted to reduce the power output by 40%, you'd simply take the power times 0.4. Now, to calculate the number of photons. You'll use the following formulas to first get the frequency (f) from the wavelength (λ), then to calculate how much energy each photon has:

$$f = \frac{c}{λ}$$

$$E = hf$$

h is Planck's constant. Now you simply take intensity (I) and divide it by the photon energy (E) to get the number of photons per unit area.

  • $\begingroup$ Do I need to divide the intensity by area of the surface to get the number of photons incident on just the surface of the metal? $\endgroup$
    – sh12
    Sep 6 at 12:36
  • $\begingroup$ @sh12 No, if you follow my steps you’ll get a result like 1x10^20 photons per square meter. To get the answer you want, more math than I know is required. I usually only do this at astronomical scales. I apologize, I misunderstood your question. $\endgroup$
    – Hokon
    Sep 6 at 18:35

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