Quantum Light

01Quantum Light

Black-body radiation

This section covers perfect black bodies, their emission spectra and how the spectrum changes as temperature increases.

Perfect black bodies

A perfect black body is an idealised body that absorbs all incident electromagnetic radiation and neither reflects nor transmits any of it.

An efficient absorber is also an efficient emitter, so a perfect black body represents an ideal emitter. The distribution of its emitted radiation with wavelength is its black-body spectrum.

Changing temperature

The emitted spectrum depends on the temperature of the body.
For a cooler body, maximum emission occurs in the infrared region.
As temperature rises, the wavelength at maximum intensity becomes shorter.
At sufficiently high temperatures, a significant amount of visible radiation is emitted.

Increasing temperature therefore changes both the intensity distribution and the position of the peak.

Exam Tip: When describing a temperature increase, state the direction of the peak shift explicitly: $\lambda_{\max}$ moves to a shorter wavelength.

02Quantum Light

Ultraviolet catastrophe

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Classical theory versus experiment

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Evidence or model	Short-wavelength behaviour
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03Quantum Light

Planck's quantisation

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Discrete energy exchange

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E=hf

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h=6.63\times10^{-34}\ \mathrm{J\,s}

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E=nhf

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04Quantum Light

Photoelectric evidence

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Why wave theory was challenged

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Observation	Problem for classical wave theory
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05Quantum Light

Einstein's photon model

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From Planck's quanta to photons

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E=hf

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Explaining the evidence

Threshold frequency

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Immediate emission

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Frequency and intensity

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Why the model was significant

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