The Photoelectric Effect
Introduced| English |
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| photoelectric effect/ˌfəʊtəʊɪˈlektrɪk ɪˈfekt/ |
Blue light knocks electrons out — red light never does, however bright
- Shine light on a metal and electrons can fly off. But there's a catch.
- Blue light ejects them instantly; red light does nothing — even blindingly bright red.
- No wave theory can explain that. Only photons can.
- This is the photoelectric effect 光电效应, Einstein's proof that light is made of particles.
One photon, one electron
- Light delivers energy in photons, each carrying $E = hf$.
- To escape the metal, an electron needs one photon with enough energy in a single hit.
- A photon below a threshold frequency simply doesn't have enough energy — no electron leaves.
- Above the threshold, each photon knocks out one electron.
The photoelectric effect is evidence that light behaves as:
A threshold frequency can only be explained if light comes in photons.
Why brightness doesn't help below threshold
- If the frequency is too low, even a very bright beam won't eject any electrons.
- More photons of too-little energy each still can't free an electron — brightness gives quantity, not quality.
- Above threshold, a brighter beam ejects more electrons (more photons), but not faster ones.
- Only raising the frequency gives each electron more kinetic energy.
Below the threshold frequency, increasing the brightness of the light:
Below threshold, each photon is too weak — brightness cannot free any electrons.
Above threshold, a brighter beam ejects:
More photons eject more electrons; speed depends on frequency, not brightness.
The energy balance
- Some photon energy goes into freeing the electron (the work function); the rest becomes its kinetic energy.
- In symbols: $hf = \phi + \text{KE}_{\text{max}}$.
- So above threshold, higher frequency → faster ejected electrons.
- This exact accounting is what earned Einstein the Nobel Prize.
The photoelectric effect
The maximum kinetic energy of ejected electrons rises linearly with the light's frequency, above a threshold.
Above the threshold, higher-frequency light ejects electrons with more kinetic energy.
$hf = \phi + \text{KE}_{\text{max}}$: more photon energy leaves more kinetic energy.
The minimum energy needed to free an electron from the metal is the ____ function.
The work function $\phi$ is the energy to free an electron; the rest becomes kinetic energy.
Select all true statements about the photoelectric effect.
A threshold frequency, particle behaviour, and frequency-set speed all hold. Bright red (below threshold) ejects nothing.
Below the threshold frequency, no electrons are ejected — no matter how bright the light. Brightness (more photons) can't substitute for frequency (energy per photon). A wave theory expects bright light to always work; the fact that it doesn't proves light is particles.
Dim ultraviolet light ejects electrons from a metal, but intense red light does not. Why?
- UV photons are above the threshold frequency (enough energy each), so they eject electrons.
- Red photons are below it — each is too weak, and adding more (brighter) doesn't help.
The photoelectric effect: light ejects electrons only above a threshold frequency, because energy arrives in photons ($E = hf$) that must free an electron one at a time. Below threshold, brightness doesn't help; above it, higher frequency gives faster electrons ($hf = \phi + \text{KE}_{\text{max}}$). It proves light is particles.