Table of Contents
The Photoelectric Effect
The photoelectric effect is the emission of electrons from a metal surface when light of sufficient frequency shines on it. Discovered experimentally by Hertz in 1887, it was explained by Einstein in 1905 using the concept of light quanta (photons). This work earned Einstein the Nobel Prize in 1921 and was crucial evidence for the quantum nature of light.
The key observations are: (1) below a threshold frequency, no electrons are emitted regardless of light intensity; (2) above the threshold, electrons are emitted immediately; (3) increasing intensity increases the number of electrons but not their maximum kinetic energy; (4) increasing frequency increases the maximum kinetic energy linearly. Classical wave theory could not explain these observations.
Einstein's Equation
Work Functions of Common Metals
| Metal | φ (eV) | λ0 (nm) |
|---|---|---|
| Cesium | 2.1 | 590 |
| Sodium | 2.3 | 539 |
| Aluminum | 4.3 | 288 |
| Gold | 5.1 | 243 |
| Platinum | 5.6 | 221 |
Frequently Asked Questions
Why can't classical physics explain the photoelectric effect?
Classical wave theory predicts that any frequency of light should eject electrons given enough intensity and time. It cannot explain the sharp frequency threshold, instantaneous emission, or the independence of electron energy from intensity. Only the photon model, where light comes in discrete quanta of energy hf, explains all observations.
What is the stopping voltage?
The stopping voltage is the minimum reverse voltage needed to prevent the most energetic photoelectrons from reaching the collector. It equals KEmax/e, where e is the electron charge. Measuring stopping voltage versus frequency gives a straight line whose slope is h/e, providing a measurement of Planck's constant.