Modern Physics
Retarding potential
Sweep V — watch the I–V curve; current falls as retarding voltage approaches V_stop.
Retarding potential — interactive Modern Physics simulation. Sweep V — watch the I–V curve; current falls as retarding voltage approaches V_stop. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Stopping potential
Sweep V — watch the I–V curve; current falls as retarding voltage approaches V_stop.
Retarding voltage opposes the fastest photoelectrons. At stopping potential V_stop, even the most energetic electrons fail to reach the collector: eV_stop = K_max = hf − φ. Measuring V_stop vs frequency yields Planck's constant and work function—the experiment Einstein explained.
- eV_stop = K_max
- I–V curve
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
A retarding voltage is applied to push ejected photoelectrons back toward the metal. What voltage is needed to just barely stop even the fastest ejected electrons?
Predictions to weigh
- The stopping voltage doesn't depend on the photon energy.
- The stopping voltage (in volts) numerically equals the maximum kinetic energy (in eV) — it takes exactly that much reverse work to stop the fastest electrons.
- The stopping voltage depends on the photocell's shape, not on the light.
Variable roles
What you set:
- Wavelength λ (nm)
What you measure:
- Photon energy E_γ (eV)
- Stopping potential V_stop (V)
How the investigation runs
- Open the photoelectric-retard preset and press Reset. The photocell's work function is fixed at 2.0 eV.
- Enable the stopping-potential readout.
- Set the light's wavelength for each trial and record the stopping potential needed to halt the fastest photoelectrons.
Governing equation
Stopping Potential — V_stop = K_max/e
The retarding voltage that just barely stops the fastest photoelectrons equals their maximum kinetic energy in eV: V_stop = K_max = E_γ − φ. This gives a direct, easily-measured readout of the photon energy above threshold.
What the printable worksheet asks students to work out
- For one trial, compute E_γ = hc/λ, then V_stop = K_max = E_γ − φ using φ = 2.0 eV (numerically, since 1 eV of kinetic energy is stopped by exactly 1 V of retarding potential). Compare to the table.
- Explain why measuring the stopping voltage is a clever experimental trick: it directly reads off K_max in eV without needing to measure electron speeds directly.
Where this shows up beyond the lab
- If you plotted V_stop against the light's FREQUENCY (not wavelength) instead, the slope of that line would equal h/e — Planck's constant divided by the electron charge. Explain why this graph was historically used to measure Planck's constant experimentally.
- What would the stopping potential be for light with photon energy below 2.0 eV? Would you even need a stopping potential in that case?
- 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 Photoelectric Retard
- Select the photocell
- Press Play
- Stopping potential
- 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.