Physics

Problem

A metallic surface of area

0.50\,\text{cm}^2

is irradiated by a monochromatic beam of wavelength

400\,\text{nm}

carrying an incident power of

5.0\,\text{mW}

. The surface emits one electron for every

50

photons incident (quantum efficiency

2\%

). a. (i) Calculate the number of photons incident per second on the surface. b. (ii) Determine the saturation photocurrent generated under these conditions. c. (iii) If the incident power is reduced to

1.0\,\text{mW}

, find the new saturation current. State what happens to the stopping potential when the power is reduced, giving a brief reason.

Intuitive explanation

This problem combines photon energy with the photoelectric effect and counting statistics. The key insight is that each photon carries energy $E = hc/\lambda$ , so you can find how many photons hit the surface per second from the incident power, then use the quantum efficiency to find the actual electron emission rate. Think of it as a two-step filter: power tells you photon flux, and efficiency tells you what fraction actually escape as electrons.

Intuitive explanation

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