Modern Physics
Photoelectric Effect Explained
How light ejects electrons from metals: Einstein's photoelectric equation, threshold frequency, work functions, worked problems and real-world uses of the photoelectric effect.
14 min read
Topic Cluster
Modern Physics
Modern physics: relativity and quantum mechanics, and the strange truths that surfaced once we looked closely enough at very fast and very small things.
What Is Modern Physics?
Modern Physics is what happens when the classical picture breaks down — at speeds close to light, and at scales close to a single atom. Relativity and quantum mechanics rebuild the fundamentals on stranger but deeper foundations.
| Relationship | Equation | What it tells you |
|---|---|---|
| Mass-energy equivalence | E = m·c² | Rest energy of a mass |
| Photon energy | E = h·f | Energy of a single quantum of light |
| De Broglie wavelength | λ = h / p | Wavelength of a moving particle |
| Lorentz factor | γ = 1 / sqrt(1 − v²/c²) | Time dilation and length contraction |
| Uncertainty principle | Δx · Δp ≥ h-bar / 2 | Position and momentum cannot both be sharp |
| Hydrogen energy levels | E = −13.6 / n² eV | Bohr model of the atom |
