The method and the results are on the photoelectric effect lab. Do the analysis yourself first, then check the answers below.
Quick answer: Light comes in photons of energy E = hf. In the photoelectric effect a photon ejects an electron only if hf is at least the work function φ, and the electron then has maximum kinetic energy Kmax = hf − φ. These answers go with the Physics 11-12 lab on photoelectric effect: the graph, the calculations, the uncertainties and the conclusion explained, for all 6 analysis questions.
Here are the photoelectric effect lab answers for physics grades 11-12: graph, calculations, uncertainties and conclusion. First do the lab yourself, then compare, and go back to the lesson if needed.
The sample data lie on a straight line with a gradient close to hc/e, giving a value of h near the accepted one. Your own value will depend on how well you judge the point at which each LED just glows, but it should be within about 10 percent.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB E.2
The line crosses the V axis at about −0.07 V, so the observed turn-on voltages are a little below hc/(eλ). An LED emits a spread of wavelengths around its quoted value, and the eye notices the faint longer-wavelength light before the LED is fully on.
Question 6 Evaluation
The judgement of when the LED just glows is subjective, and the LED wavelengths are only nominal values. Use a light sensor or a photodiode to detect the onset, and check the wavelengths with a spectrometer or diffraction grating.
