# Photoelectric Effect Virtual Lab - Stopping Voltage, I–V Curve, Measure Planck's Constant

> Virtual photoelectric lab: choose a metal, wavelength, intensity and bias, watch photoelectrons and the I–V curve, and fit Planck's constant.

- URL: https://qilingtools.com/en/physics/photoelectric
- Category: Physics Tools (https://qilingtools.com/en/physics)
- Tags: [Physics](https://qilingtools.com/en/tags/physics), [Education](https://qilingtools.com/en/tags/education)
- Last updated: 2026-10-29
- Price: Free, no sign-up
- Privacy: Runs locally in the browser; files are never uploaded

Run a complete photoelectric experiment in the browser. Choose a cathode metal such as caesium, sodium or zinc, set the light wavelength, power and anode voltage, and watch photons knock out electrons that are then accelerated or pulled back by the field while the ammeter reads the current. The Millikan tab lets you measure stopping voltages for the mercury lines and fit a least-squares line to get Planck's constant, and a final tab contrasts classical wave theory with what is observed.

## How to use

1. In "Phototube lab" choose a cathode metal and drag the wavelength, light power and anode voltage while watching the electrons and the current
2. Make the voltage negative to find the stopping voltage where the current just reaches zero; keep the I–V curve and compare another wavelength or intensity
3. Open "Millikan: measure h" and measure the stopping voltage for each mercury line, or all five at once
4. Read h, the work function and the threshold frequency from the fitted U_s–ν line, with the error versus the accepted value
5. In "Classical vs quantum" change the intensity and compare the classical emission delay with what experiments show

## Features

- Work functions and threshold wavelengths for 15 metals, from visible to ultraviolet
- Photon and photoelectron animation: electrons slow and turn back under retarding bias, and are collected under forward bias
- I–V characteristic with saturation current and stopping voltage; keep several curves to compare
- Photon rate, emitted electrons per second and saturation current from light power and quantum efficiency
- Millikan experiment with noisy simulated readings and a least-squares fit of h with its uncertainty
- Side-by-side classical vs photon predictions, including the classical emission delay

## FAQ

### Why does bright red light eject no electrons while weak ultraviolet does?

Light comes in photons whose energy E = hν depends only on frequency. An electron absorbs one photon at a time and can escape only if hν exceeds the work function φ. Intensity just sets how many photons arrive per second, not the energy of each, so below the threshold frequency nothing is emitted however bright the light, while ultraviolet photons have enough energy to eject electrons immediately even in weak light, only fewer of them.

### How is the stopping voltage related to the maximum kinetic energy?

With a retarding voltage U on the anode an electron must climb a potential energy eU to reach it. When eU equals the maximum kinetic energy K_max, even the fastest electrons are turned back and the current drops to zero. That voltage is the stopping voltage: eU_s = K_max = hν − φ, so it depends only on the frequency and the metal, not on intensity.

### How does Millikan's experiment measure Planck's constant?

Rewrite eU_s = hν − φ as U_s = (h/e)ν − φ/e. Measuring the stopping voltage at several frequencies and plotting U_s against ν gives a straight line whose slope is h/e, so with the electron charge known you get h. The intercept gives the work function but also includes the contact potential between the electrodes; that only shifts the line, not its slope, so h is still measured accurately.

### What determines the saturation current?

At a large enough forward voltage almost every emitted electron is collected and the current stops rising. That saturation current is I_s = e·η·P/(hν): proportional to the light power P and the quantum efficiency η, i.e. the number of electrons ejected per second. At the same power, shorter wavelengths deliver fewer photons but usually have higher quantum efficiency.

### Why can classical wave theory not explain the photoelectric effect?

Classically the light energy is spread over the wavefront and an electron must slowly accumulate it. For one atomic cross-section, 1 W/m² would take about ten seconds and weak light days, yet electrons appear essentially instantly. Classical theory also predicts that kinetic energy grows with intensity and that any frequency works, both contrary to experiment. Einstein's light-quantum hypothesis explains all of these at once.

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