Quanta: Where the Smooth World Turns Grainy
Shine light on metal and electrons pop out — but only if the colour is right, no matter how bright the beam. Explaining that one stubborn fact broke classical physics.
Builds on: 5.3 Colour & the Spectrum: Most Light Is Invisible2.1 Energy: The Quantity That Only Changes Costume
An experiment that shouldn’t have mattered
Light falling on a metal can kick electrons off its surface — the photoelectric effect. Classically, light is a wave delivering energy continuously, so brighter light should kick harder, and even dim light should manage eventually. The metal disagreed on every count:
- Below a threshold frequency, nothing — however blinding the beam, however long you wait.
- Above it, electrons leave instantly, even in the dimmest light.
- Brighter light: more electrons, but not faster ones. Only bluer light makes faster ones.
Einstein’s uncomfortable answer
In 1905 Einstein took Planck’s bookkeeping trick literally: light arrives in indivisible packets — photons — each carrying an energy set only by frequency:
Now the stubborn facts are obvious. An electron is freed by absorbing one photon; if that photon’s E is below the escape cost (the work function), nothing happens — a million weak packets don’t pool. Brightness is just packet count: more electrons, same energy each. Red light is cheap coins, ultraviolet is high-denomination notes, and the metal is a vending machine that takes exact change only. This — not relativity — is what Einstein’s Nobel Prize cites.
Matter is grainy too
The graininess spreads. Atoms hold electrons only on discrete energy rungs — which is why heated elements emit sharp spectral lines (last lesson’s fingerprints): each line is a photon of exactly one rung-to-rung drop. A neon sign’s orange and a sodium lamp’s yellow are quantum leaps you can see from the street. Yet waves refuse to leave the story: streams of single electrons build up interference patterns, and Unit 4’s wave-test says wave while the clicks of detectors say particle. Both, in honesty, are true — the quantum world’s standing offer of humility.
Solar panels are the photoelectric effect monetised — photons promoting electrons into a current. Camera sensors count photons; LEDs run the effect in reverse, each electron dropping a rung and paying out one photon whose colour is the rung height. E = hf is consumer electronics now.
⚗️ Lab — The Photoelectric Bench
A metal plate, a light source with frequency and brightness dials, an electron counter.
- Start deep in the red and crank the brightness to maximum: nothing. Not one electron.
- Now slide the frequency up — at the threshold, electrons instantly, even at minimum brightness.
- Above threshold, compare the dials: brightness moves the count; frequency moves the speed.
Work it by hand — 0 / 1
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±3% count.