Why does no current flow when I use long-wavelength (red) light, even at very high intensity?

This is the central quantum surprise. Electron emission requires a single photon to have enough energy to overcome the metal's work function. Long-wavelength photons have low energy (E = hc/λ). If this energy is below the work function, no single photon can eject an electron, no matter how many photons (high intensity) are present. This contradicts classical wave theory, which predicts that enough wave energy should always eventually free an electron.

What exactly is the stopping potential, and what does measuring it tell us?

The stopping potential (V_s) is the reverse voltage applied between the electrodes that is just sufficient to stop the most energetic photoelectrons from reaching the collector, reducing the photocurrent to zero. Since the work done by the voltage on an electron is eV_s, measuring V_s directly gives the maximum kinetic energy: K_max = eV_s. This provides an experimental way to measure how photon energy (hf) and work function (φ) relate, verifying Einstein's equation.

How does changing the metal target affect the experiment?

Different metals have different work functions (φ). A metal with a larger φ requires higher-energy photons (shorter wavelength/higher frequency) to initiate emission, shifting the threshold frequency. For the same incident light frequency, a metal with a larger φ will yield photoelectrons with lower maximum kinetic energy, as K_max = hf - φ. This is why materials like cesium (low φ) are used in practical light-sensing devices.

Does the simulator show photocurrent versus retarding voltage and versus frequency?

Yes. Alongside I(λ) and I(intensity), the page now plots an ideal I(V) retarding curve (monoenergetic K_max), I(ν), and K_max(ν). The retarding-voltage graph is a flat plateau at the saturation current until V reaches the stopping potential V_s = K_max/e, then drops to zero — illustrating how a reverse voltage selects against slower electrons and ultimately blocks all collected current. The K_max versus ν plot is a straight line with slope Planck’s constant h (in eV·s) and x-intercept at the threshold frequency ν₀ = φ/h.

Photoelectric Effect

The photoelectric effect demonstrates the particle nature of light, where photons incident on a metal surface can liberate electrons. This simulator models the core experimental relationships. It is governed by Einstein's photoelectric equation: K_max = hf - φ = hc/λ - φ, where K_max is the maximum kinetic energy of the ejected photoelectrons, h is Planck's constant, f (or c/λ) is the photon frequency (wavelength), and φ is the work function of the metal—the minimum energy needed to free an electron. The simulator visualizes how varying the incident light's wavelength (or frequency) and intensity affects electron emission. A key prediction is that if the photon energy hf is less than φ, no electrons are emitted regardless of light intensity, establishing the concept of a threshold frequency. For photons above this threshold, K_max depends linearly on frequency, not on intensity. The model also connects K_max to the stopping potential V_s via eV_s = K_max, showing how a reverse voltage can halt the photocurrent. Students can explore how changing the metal (and thus φ) shifts the threshold. The simulator simplifies reality by assuming a single work function, ideal monochromatic light, and an idealized vacuum environment with no electron scattering or thermal effects. By interacting, learners directly test the quantum principles that classical wave theory fails to explain, solidifying their understanding of photon energy, quantization, and the experimental evidence for light's particle behavior.

Who it's for: High school and introductory undergraduate physics students studying modern physics, quantum mechanics, or the particle nature of light. It is also valuable for educators demonstrating the historical and conceptual foundations of quantum theory.

Key terms

Photoelectric Effect
Work Function
Photon
Stopping Potential
Threshold Frequency
Planck's Constant
Maximum Kinetic Energy
Quantum Mechanics

Live graphs

How it works

Light on a metal ejects electrons only when photon energy exceeds the work function φ. The threshold wavelength is λ₀ = hc/φ and frequency ν₀ = φ/h. Above threshold, maximum kinetic energy grows with frequency (K_max = hν − φ) while the photocurrent scales with light intensity — a simple classical wave picture cannot explain the threshold.

Key equations

E = hν = hc/λ (here E in eV, λ in nm, hc ≈ 1240 eV·nm)

K_max = E − φ when E > φ ; V_s = K_max / e

Frequently asked questions

Why does no current flow when I use long-wavelength (red) light, even at very high intensity?: This is the central quantum surprise. Electron emission requires a single photon to have enough energy to overcome the metal's work function. Long-wavelength photons have low energy (E = hc/λ). If this energy is below the work function, no single photon can eject an electron, no matter how many photons (high intensity) are present. This contradicts classical wave theory, which predicts that enough wave energy should always eventually free an electron.
What exactly is the stopping potential, and what does measuring it tell us?: The stopping potential (V_s) is the reverse voltage applied between the electrodes that is just sufficient to stop the most energetic photoelectrons from reaching the collector, reducing the photocurrent to zero. Since the work done by the voltage on an electron is eV_s, measuring V_s directly gives the maximum kinetic energy: K_max = eV_s. This provides an experimental way to measure how photon energy (hf) and work function (φ) relate, verifying Einstein's equation.
How does changing the metal target affect the experiment?: Different metals have different work functions (φ). A metal with a larger φ requires higher-energy photons (shorter wavelength/higher frequency) to initiate emission, shifting the threshold frequency. For the same incident light frequency, a metal with a larger φ will yield photoelectrons with lower maximum kinetic energy, as K_max = hf - φ. This is why materials like cesium (low φ) are used in practical light-sensing devices.
Does the simulator show photocurrent versus retarding voltage and versus frequency?: Yes. Alongside I(λ) and I(intensity), the page now plots an ideal I(V) retarding curve (monoenergetic K_max), I(ν), and K_max(ν). The retarding-voltage graph is a flat plateau at the saturation current until V reaches the stopping potential V_s = K_max/e, then drops to zero — illustrating how a reverse voltage selects against slower electrons and ultimately blocks all collected current. The K_max versus ν plot is a straight line with slope Planck’s constant h (in eV·s) and x-intercept at the threshold frequency ν₀ = φ/h.