Physics

Problem

Monochromatic light of wavelength

310\,\text{nm}

and power

12\,\text{mW}

is uniformly incident on a photoemissive surface of area

2.0\,\text{cm}^2

. The quantum efficiency of the surface is

25\%

, and every emitted electron is collected. Calculate the saturation current produced. (Use

h = 6.63 \times 10^{-34}\,\text{J}\,\text{s}

c = 3.0 \times 10^{8}\,\text{m}\,\text{s}^{-1}

e = 1.6 \times 10^{-19}\,\text{C}

Intuitive explanation

The key is to connect photon arrival rate to electron emission rate via quantum efficiency, then convert that to current. First, find how many photons hit the surface per second using the power and energy per photon ( $E = hf = hc/\lambda$ ). Then apply the 25% quantum efficiency to get the actual number of electrons emitted per second. Finally, convert electrons per second to current using $I = ne$ , where $n$ is the electron flow rate.

Intuitive explanation

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