A Real Photocell: Sodium's Work Function
1 · Predict
Sodium's work function is 2.28 eV, corresponding to a threshold wavelength around 544 nm (yellow-green light). Would red light (around 650 nm) eject any photoelectrons from sodium, no matter how bright it is?
- No — red light's photon energy is below sodium's work function, so no electrons are ejected regardless of intensity.
- Yes, if the light is bright enough.
- It depends on how long the light shines.
2 · Set Up
- Open the sodium-photocell preset and press Reset. Sodium's work function is 2.28 eV.
- Enable the photon-energy and photoemission-status readouts.
- Set the light's wavelength for each trial and record the maximum kinetic energy (0 if below threshold).
3 · Collect Data
| Wavelength λ (nm) | Photon energy E_γ (eV) | Max kinetic energy K_max (eV) |
|---|---|---|
| 450 | ||
| 500 | ||
| 550 |
Plot K_max (y-axis) against wavelength λ (x-axis) for your three trials. At what wavelength does K_max hit zero?
4 · Analyze
- For one trial, compute E_γ = hc/λ, then K_max = max(0, E_γ − φ) using φ = 2.28 eV. Compare to the table — confirm the longest-wavelength trial gives K_max = 0.
- Sodium's threshold wavelength is λ_threshold = hc/φ ≈ 544 nm. Explain why any wavelength longer than this (lower photon energy) can never eject an electron from sodium, no matter the light's intensity.
5 · Extend
- Different photovoltaic materials are chosen to have work functions matched to the solar spectrum. Explain why a material with too HIGH a work function would waste most of the sun's visible light instead of converting it to electricity.
- This experiment shows K_max depends on wavelength (color), not brightness. What physical property of the light DOES change if you increase brightness at a fixed wavelength above threshold?
The Physics Behind This Experiment
Threshold Behavior
K_max = max(0, E_γ − φ): below the work function, no photoelectrons are emitted regardless of intensity — a sharp threshold that a purely classical (wave) picture of light cannot explain.