Quanta of light ( the smallest packet of energy

Planck’s work was revolutionary, but even Planck was not sure what it meant and tried for years to reconcile quantization with classical ideas. Moreover, Planck had quantized the oscillators emitting the radiation, not the radiation itself. He assumed that the radiation could be described by Maxwell’s equations. After all, these had been supported by the experiments of Hertz and others and their continuous fields seemed consistent with the continuous wave model of light established by Christiaan Huygens and borne out by experiments involving interference and diffraction (such as Thomas Young’s double-slit experiment).

Perhaps Einstein had less to risk by approaching things from a completely new direction. In 1905 he was employed in the patent office in Bern as a ‘technical expert third class’, but he had a firm grounding in classical physics and was busy producing his own revolution. In this year he published three major papers, each made a crucial contribution to a fundamental area of theoretical physics and any one of them would have been sufficient to establish him as one of the leading physicists of his time. One explained Brownian motion and effectively ended the debate over the ‘reality’ of atoms. A second contained the special theory of relativity. The third concerned the nature of light.

In it Einstein suggested that there might be some phenomena which cannot be explained using the continuous variables of classical electromagnetism, phenomena for which a particle model might work better. It is not clear to what extent he was influenced by Planck’s work, but he was certainly aware of it. They beauty of Einstein’s approach is that he managed to look directly at physical processes such as emission and absorption of light and built up an explanation from scratch. It is hard to imagine Planck doing this. Here are some lines from the introduction to Einstein’ great paper.

” it should be kept in mind that optical observations refer to values averaged over time and not to instantaneous values. Despite the complete experimental verification of the theory of diffraction, reflection, refraction, dispersion and so on, it is conceivable that a theory of light operating with Continuous three dimensional functions will lead to conflicts with experience if it is applied to the phenomena of light generation Conversion,”

Planck had considered the oscillators that emit black-body radiation. Einstein went for the radiation itself. He imagined an enclosure containing electromagnetic radiation in thermal equilibrium with the walls. He had a firm grounding in the statistical thermodynamics of Boltzmann and applied this to the radiation. He examined the way the energy and entropy of the radiation changed as the volume and temperature of the enclosure changed, and found that they behaved in a very similar way to the energy and entropy of a gas. It was as if the radiation itself consisted not of continuous waves but of particles bouncing about, and the energy transferred by one of these particles was
given by :


an equation reminiscent of Planck’s results. Much later these ‘particles’ would be called ‘photons’ but we shall adopt the term from now on. (You may be surprised to learn that the term ‘photon’ was not used until 1926. This was partly because of the unease many physicists felt at resurrecting the discredited particle model for light. The crucial experiments that showed that photons could scatter off electrons like classical particles, and would conserve energy and momentum when they did so, were carried out by Compton and his co-workers in 1923-5) .

Einstein suspected that this particle model would be important when radiation is emitted and absorbed and showed that it worked well in three particular cases:

• photoluminescence (emission of light by a material which has previously absorbed light); photoionisation of gases (ionisation by light);

• photoelectric effect (ejection of electrons from the surface of a metal when illuminated by light).

In all three cases the particle model worked much better than the continuous wave model and Einstein won the 1921 Nobel Prize “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect”.

Plank was first to introduce quantization but Einstein’s work really made physicist take quantization seriously. It also undermined the classical view of reality, a consequence that Einstein himself found difficult to deal with.

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