Physic Labs

Problem 3

Monochromatic light of wavelength λ=400\lambda=400 nm illuminates a metal surface with work function W=2.00W=2.00 eV. Use hc=1240hc=1240 eV nm and electron charge magnitude ee. Assume sufficient intensity and photoelectrons emitted in all directions. (a) Calculate the photon energy and maximum photoelectron kinetic energy. (b) Find the stopping potential needed so no photoelectron reaches the collector. (c) For the same metal, calculate the longest wavelength that still produces photoelectrons. (d) Change the light to λ=300\lambda=300 nm and find the new maximum kinetic energy.
problems.proof.stepOf
problems.proof.prevStepproblems.proof.nextStep
eVs=Kmax⁡⇒Vs=1.10 eVe=1.10 VeV_s=K_{\max}\Rightarrow V_s=\frac{1.10\ \mathrm{eV}}{e}=1.10\ \mathrm V
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Apply a retarding potential such that the field's work on an electron, eVseV_s, equals Kmax⁡K_{\max}. Since the energy is expressed in electronvolts, the potential is the same number in volts.

problems.proof.pitfall. The stopping potential depends on light frequency, not intensity; do not adjust VsV_s according to how many electrons arrive.