Modern Physics
Photoelectric threshold
Tune λ and φ — photocurrent stops below the threshold frequency; read V_stop.
Demonstrate the photoelectric effect and Einstein's photon model. Find the threshold frequency and stopping potential for different metal surfaces.
Photoelectric effect
Tune λ and φ — photocurrent stops below the threshold frequency; read V_stop.
Einstein's photoelectric equation K_max = hf − φ links photon energy to ejected electron kinetic energy. Below threshold frequency hf < φ, no electrons leave regardless of intensity—proof light is quantized. Adjust wavelength and work function to find cutoff.
- K_max = hf − φ
- Threshold frequency
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
Light of different wavelengths hits a metal with work function 2.3 eV. As long as both wavelengths are above the threshold, does a bluer (shorter-wavelength) photon eject an electron with more kinetic energy?
Predictions to weigh
- No — kinetic energy depends only on light intensity, not wavelength.
- It depends on the metal's color, not the light's wavelength.
- Yes — above threshold, shorter wavelength means higher photon energy, so ejected electrons have more kinetic energy.
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 photoelectric-threshold preset and press Reset. The photocell's work function is fixed at 2.3 eV.
- Enable the photon-energy and maximum-kinetic-energy readouts.
- Set the light's wavelength for each trial and record the maximum kinetic energy of ejected electrons.
Governing equation
Photon Energy — E = hc/λ
A photon's energy depends only on its wavelength (or frequency): E = hc/λ = hf. Shorter wavelength means higher-energy photons.
Maximum Photoelectron Kinetic Energy — K_max = E − φ
Einstein's photoelectric equation: an incoming photon's energy first pays the work function (the energy binding an electron to the metal), and whatever's left over becomes the ejected electron's kinetic energy: K_max = E_γ − φ.
What the printable worksheet asks students to work out
- For one trial, compute E_γ = hc/λ using h = 6.626×10⁻³⁴ J·s, c = 3×10⁸ m/s, then K_max = E_γ − φ using φ = 2.3 eV. Compare both to the table.
- Explain why K_max increases linearly with photon energy (and therefore with 1/λ) once you're above threshold, with a slope of exactly 1 and a y-intercept of −φ.
Where this shows up beyond the lab
- Classical wave theory predicted that even dim, low-frequency light should eventually eject electrons if you wait long enough or turn up the intensity. Explain why the photoelectric effect's sharp energy threshold (independent of intensity) was so surprising before Einstein's photon explanation.
- If you doubled the light's intensity at the same wavelength, would K_max change? What would change instead?
- 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
- More or less energy?
- Shift toward blue
- Capture the kinetic energy
- Open the data
- Explain your evidence
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.