4.10.1Electrons, Waves and Photons
The Photoelectric Effect
5 min read 5 parts Lesson 13 of 62 Unit quiz Virtual lab
Part 1 of 5What Happens
Emission of electrons
- When electromagnetic radiation of suitable frequency hits a metal surface, electrons are emitted from the surface.
Photoelectrons
- The electrons that leave the metal in this way are called photoelectrons.
Threshold frequency
- Emission happens only if the radiation frequency is at or above a minimum value that depends on the metal.
Immediate emission
- Photoelectrons leave as soon as suitable radiation arrives, with no measurable delay even at very low intensity.
Part 2 of 5Why the Wave Model Fails
Why the Wave Model Fails
Prediction: energy builds up
- A wave model says a dim wave would slowly give an electron enough energy, so there should be a delay.
Observation: no delay
- Photoelectrons are emitted at once, even from very dim light of high enough frequency.
Prediction: any frequency works
- A wave model says a bright enough beam of any frequency should eventually free electrons.
Observation: threshold
- Below the threshold frequency no electrons are emitted however intense the radiation is.
| Observation | Wave model predicts | What is seen |
|---|---|---|
| Low intensity | Delay before emission | Immediate emission |
| Low frequency, high intensity | Electrons emitted | No emission |
| Higher intensity | Greater electron energy | Same maximum energy, more electrons |
Part 3 of 5The Photon Explanation
Photon
- Electromagnetic radiation is carried in packets of energy called photons, and each photon carries energy hf.
One-to-one
- A surface electron absorbs the energy of one photon only, and it cannot store up energy from several photons.
Enough energy
- If one photon carries less energy than the work function, no electron can escape, whatever the number of photons.
Number of electrons
- More photons per second means more electrons freed per second, so intensity affects the number and not the energy.
Part 4 of 5Developing the Quantum Idea
1
Classical physics described light purely as a wave, supported by diffraction and interference.
2
Planck proposed that energy is exchanged in discrete amounts to explain radiation from hot objects.
3
Einstein used the idea of photons to explain the photoelectric effect.
4
Bohr used quantised energy levels to explain the line spectrum of atoms.
5
The photoelectric effect therefore provides evidence for the particle nature of light, alongside the wave evidence.
Part 5 of 5Exam Checklist & Tips
4.10.1 – Key Points for the Exam
- Photoelectric effect The emission of electrons from a metal surface when electromagnetic radiation shines on it.
- Photoelectron An electron emitted from a surface by the photoelectric effect.
- Photon A packet of electromagnetic energy equal to hf.
- Threshold frequency The minimum frequency of radiation that can release a photoelectron from a given metal.
- Quantised Existing only in discrete amounts.
Common Mistakes
- Saying brighter light gives photoelectrons more energy;
- it gives more photoelectrons but not more energy each.
- Forgetting that each electron interacts with a single photon only.
- Claiming the wave model explains the threshold frequency; it does not.
- Describing emission as delayed for low intensity;
- below threshold nothing happens, above it emission is immediate.