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Planck's quantum hypothesis
Beginner
Energy is emitted in discrete packets.
E = hfPlanck proposed that oscillators in a cavity exchange energy only in discrete amounts E = hf, which reproduced the blackbody curve exactly. He didn't initially believe light itself was quantised.
A 500 nm photon has E = 2.48 eV.
Higher frequency means a larger energy quantum.
In more detail
Planck proposed in 1900 that energy is emitted and absorbed in discrete packets, or quanta, with energy proportional to frequency. This resolved the ultraviolet catastrophe in blackbody radiation.
Real-world uses
- Planck's idea is the basis of all quantum physics.
- Thermal imaging uses the blackbody spectrum described by Planck.
- Engineers designing lasers and LEDs rely on the fact that light is emitted and absorbed in discrete packets of energy.
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Frequently asked questions
Was Planck a revolutionary?
He was initially reluctant to accept quantisation, seeing it as a mathematical device.
What is a quantum?
A discrete unit of energy, such as the energy of a photon.
What is a quantum of energy?
The smallest packet of energy at a given frequency, which is Planck's constant times the frequency.
Related topics in Quantum Mechanics
Failure of classical physics · Planck's constant · Photoelectric effect · Einstein's photoelectric equation