Photoelectric effect (3D lab)
About this app
The photoelectric effect in a 3D lab: filter Hg lines, set the intensity at a grey wedge, drag the retarding voltage. Intensity changes only the current, frequency changes the stopping voltage. With the Hallwachs experiment, the retarding-field method, h from the line and the wave model for comparison.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
What happens in the photoelectric effect
In the photoelectric effect, light frees electrons from a metal. Einstein explained this in 1905 with light quanta: a photon gives its energy h·f entirely to one electron. Part of it is the work function W, the rest remains as kinetic energy: E_k = h·f − W. This is why there is a threshold frequency below which no electron leaves, and why the intensity of the light changes only the number of electrons.
What the app shows
A mercury lamp, a filter wheel with the lines 365, 405, 436, 546 and 578 nm, a grey wedge and a vacuum photocell stand on an optical bench. Photons fly to the cathode as wave packets, electrons run through the retarding field to the anode or turn back. There are also the I–U curve, the E_k–f line and the Hallwachs experiment with zinc plate, electroscope and glass plate. A comparison sets the wave model against the photon model.
Try it yourself
First predict what happens to the stopping voltage when you triple the intensity. Then record two curves and compare them. Change the line and the cathode material, drag the retarding voltage until the current reaches zero, and collect data points. The slope of the line gives h, the intercept gives the work function. Exam tasks with random values show the worked solution.
Frequently asked questions
- Why does brighter light not make the electrons faster?
- Brighter light means more photons per second, but each photon still carries the energy h·f. An electron receives the energy of exactly one photon. So intensity raises only the current, while the maximum kinetic energy and the stopping voltage stay the same.
- What is the stopping voltage in the photoelectric effect?
- The stopping voltage V₀ is the retarding voltage at which even the fastest electrons no longer reach the anode and the current falls to zero. It follows that e·V₀ = E_k,max = h·f − W.
- How do you find Planck’s constant from the photoelectric effect?
- Measure the stopping voltage at several frequencies and plot e·V₀ against f. The points lie on a straight line. Its slope is h, its intercept is −W and its zero crossing is the threshold frequency.
- What can I change in the app?
- You pick the spectral line on the filter wheel, set the intensity at the grey wedge, drag the retarding voltage and switch between caesium, potassium, sodium and zinc. There is also the Hallwachs experiment, prediction questions and calculation tasks.
Subject: School physics
More from School physics
- The exposure triangle
- Density and buoyancy
- The Doppler effect
- Additive and subtractive colour mixing
- Franck–Hertz tube (3D)
- Newton’s cradle and collisions
- Particle in a box (3D)
- Converging lenses and ray diagrams
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