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Photoelectric effect
Beginner
Light ejects electrons from a metal only above a threshold frequency.
photoLight of sufficiently high frequency ejects electrons from a metal immediately; below a threshold frequency no electrons come out regardless of intensity. Classical waves couldn't explain this.
UV ejects electrons from zinc; bright red light doesn't.
Intensity changes the number of electrons, not their maximum energy.
In more detail
In the photoelectric effect, light shining on a metal ejects electrons only if its frequency exceeds a threshold, regardless of intensity. This could not be explained by waves and supported the photon idea.
Real-world uses
- Solar cells and photodiodes use the photoelectric effect.
- Light meters and image sensors detect light through photoelectric processes.
- Solar cells and light sensors work because photons can knock electrons out of a metal or semiconductor.
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Frequently asked questions
Why does intensity not matter for ejection?
Each electron absorbs one photon, so ejection depends on the photon energy, which is set by frequency.
Who explained it?
Einstein in 1905, which earned him the Nobel Prize.
Why does brighter light not eject faster electrons?
Brightness changes the number of photons, not their energy, so the speed of each electron depends on the photon frequency.
Related topics in Quantum Mechanics
Failure of classical physics · Planck's quantum hypothesis · Planck's constant · Einstein's photoelectric equation · Threshold frequency · Stopping potential