Modern Physics
Sodium photocell
Configured 500 nm photons (2.48 eV) exceed 2.28 eV; 589 nm sodium D-line photons (~2.1 eV) do not emit in this model.
Sodium photocell — interactive physics simulation. Configured 500 nm photons (2.48 eV) exceed 2.28 eV; 589 nm sodium D-line photons (~2.1 eV) do not emit in this model.
Photocell I–V
500 nm (2.48 eV) emits; 589 nm (~2.1 eV) does not.
Configured 500 nm photons (2.48 eV) exceed 2.28 eV; 589 nm sodium D-line photons (~2.1 eV) do not emit in this model.
- φ for sodium
- Yellow line λ
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
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?
Predictions to weigh
- Yes, if the light is bright enough.
- It depends on how long the light shines.
- No — red light's photon energy is below sodium's work function, so no electrons are ejected regardless of intensity.
Variable roles
What you set:
- Wavelength λ (nm)
What you measure:
- Photon energy E_γ (eV)
- Max kinetic energy K_max (eV)
How the investigation runs
- 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).
Governing equation
Threshold Behavior — K_max = E − φ
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.
What the printable worksheet asks students to work out
- 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.
Where this shows up beyond the lab
- 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?
- AP Physics 2 — Unit 15: Modern Physics
- IB Physics — E.2 Quantum physics
- General High School Physics — Modern physics intro
- NGSS High School Physics — Wave-particle duality of light
- Welcome to Sodium Photocell
- Select the photocell
- Press Play
- Sodium work function
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.