Photoelectric effect
Light above a threshold frequency knocks electrons out of a metal. Maximum kinetic energy = photon energy minus the work function.
Last updated
Analogy
A coin toss from a well: you need a minimum throw before anything escapes.
Example
KE max = hf - work function= 3.1 - 2.0= 1.1 eV
In practice
400 nm light (3.1 eV) on metal with a 2.0 eV work function gives electrons up to 1.1 eV.
Practice
Flashcards
- What does the photoelectric effect show?Light behaves as photons. Energy depends on frequency, not brightness.
Interview questions
- Explain the photoelectric effect and why it could not be explained by wave theory.
- Threshold frequency exists
- Brightness does not change electron energy
- Photons carry energy hf
Questions people ask
Why does brightness not matter below threshold?
Each electron absorbs a single photon in one go. If one photon lacks the energy to free it, a thousand more photons will not help, since electrons do not store energy over time. Brightness changes how many electrons are freed, not their individual energies.
What is the work function?
The work function is the minimum energy needed to free an electron from a metal's surface. It differs by metal: about 2.3 eV for sodium and 4.7 eV for copper. A photon must carry at least this energy to eject an electron.