Physics

Physics

Problem

A 10 mW10\,\text{mW} violet laser (λ=400 nm\lambda = 400\,\text{nm}) is allowed to fall on a photo-cathode for 2 s2\,\text{s}. Assuming each photon ejects one electron, the total number of photoelectrons emitted is closest to a) 2×10162\times10^{16} b) 4×10174\times10^{17} c) 4×10164\times10^{16} d) 4×10154\times10^{15}
Intuitive explanation

The photoelectric effect is about converting light energy into individual electron ejections—one photon, one electron. Your job is to find how many photons hit the cathode, which equals the number of electrons released. Start by calculating the energy per photon using E=hf=hc/λE = hf = hc/\lambda, then figure out the total energy delivered by the laser over 2 seconds, and divide to get your photon count.

Intuitive explanation

The photoelectric effect is about converting light energy into individual electron ejections—one photon, one electron. Your job is to find how many photons hit the cathode, which equals the number of electrons released. Start by calculating the energy per photon using E=hf=hc/λE = hf = hc/\lambda, then figure out the total energy delivered by the laser over 2 seconds, and divide to get your photon count.

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