About this experiment
When light shines on a clean metal, it can knock electrons out of the surface. The surprise, explained by Einstein in 1905, is that the colour of the light decides whether this happens, not its brightness. Light arrives in packets called photons, each with energy hf. A photon can free an electron only if its energy is bigger than the work function of the metal, and whatever is left over becomes the kinetic energy of the electron. Change the wavelength, the brightness, the metal and the voltage, and measure the stopping voltage to find Planck's constant yourself.
Learning goals
- Describe the photoelectric effect and the idea of a threshold frequency
- Use E = hf to find the energy of a photon from its frequency or wavelength
- Apply the photoelectric equation K_max = hf - φ
- Explain why the stopping voltage depends on frequency but not on intensity
- Explain why the photoelectric effect supports the photon model of light
How to use it
- 1With sodium and 400 nm light, watch the electrons. Now slowly increase the wavelength. At what wavelength do they stop?
- 2Set a wavelength that frees electrons and change the intensity. What changes: the number of electrons, their speed, or both?
- 3Make the collector voltage more and more negative until the current just reaches zero. Compare this stopping voltage with the maximum kinetic energy.
- 4Measure the stopping voltage at 250, 300, 350 and 400 nm. Plot it against frequency and find the gradient.
- 5Switch to Energy and frequency. Why do all the lines have the same slope?
- 6Try zinc with visible light, then with ultraviolet. Which metals would work in a light meter for visible light?
The model
Assumptions
- Each photon frees at most one electron, and a fixed small share of the photons above the threshold frees an electron, so the saturation current is proportional to intensity.
- Freed electrons have kinetic energies spread evenly between zero and K_max, so the current falls in a straight line as the stopping voltage increases.
- Both plates are in a vacuum. The contact potential between the two metals and any current at positive voltages above saturation are ignored.
- Work functions are typical values for clean surfaces; real surfaces vary by a few tenths of an electronvolt.
Try this in class
- Q1Why does a very bright red lamp free no electrons from zinc, while a weak ultraviolet lamp does?
- Q2Light of wavelength 300 nm falls on calcium (φ = 2.87 eV). What is the stopping voltage?
- Q3If the intensity is doubled, what happens to the saturation current and to the stopping voltage?
- Q4Why do the electrons not all have the same kinetic energy, even with light of one wavelength?
- Q5The wave model predicts that dim light would take time to build up enough energy to free an electron. Why is no such delay observed?
Frequently asked questions
What is the work function?
It is the minimum energy needed to pull an electron out of the surface of a metal. Electrons deeper inside the metal need more energy, which is why the freed electrons have a range of kinetic energies up to K_max.
How can I find Planck's constant with this lab?
Measure the stopping voltage for several wavelengths, convert the wavelengths to frequencies and plot eV against f. The gradient of the straight line is h, about 4.14 × 10⁻¹⁵ eV s or 6.63 × 10⁻³⁴ J s.
Why is the current the same for all positive voltages?
Once the collector is positive, every freed electron reaches it, so the current is limited by how many electrons are freed each second. That number depends on the intensity of the light, not on the voltage.