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Photoelectric effect and quanta.

In 1905 the photoelectric effect, as observed by Lenard (see 1902), was combined with quantum theory (see 1900) by Einstein.

He showed that if light consisted of quanta, with energies proportional to frequency (inversely proportional to wavelength), then the atoms in a metal surface could only absorb intact quanta. Furthermore, long-wavelength quanta would not supply enough energy to eject an electron from the metal, no matter how intense the light might be. But as wavelength grew shorter, energy quanta grew larger, and a point would be reached where the energy was just sufficient to eject an electron. The shorter the wavelength beyond that, the more energetic the ejected electron would be and the more speedily it would travel.

Since some metals may hold electrons more firmly than others, the critical wavelength must be shorter in some cases than in others.

Einstein's analysis had several consequences.

1. It explained the photoelectric effect completely. There have not had to be any extensions or additions to the explanation since.

2. It made use of quantum theory to explain a phenomenon unexplainable otherwise, and a phenomenon moreover that had not been in Planck's mind when he worked out the theory. If quantum theory were simply a mathematical device needed to make black-body radiation come out right, it wouldn't be likely to be applicable without modification to an altogether different phenomenon.

Einstein's work therefore established quantum theory as legitimate and not merely a mathematical trick.

3. It showed that light could be treated as particles in some respects.

Newton's particles and Huygen's waves (see 1678) were thus combined into a whole that was far more complex and useful than could have been imagined on the basis of seventeenth-century knowledge. The particle aspects of light, and of electromagnetic radiation generally, are referred to nowadays as photons.

It was for this work on the photoelectric effect (not on his still greater discoveries in connection with relativity) that Einstein gained a Nobel Prize in physics in 1921.

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Author:Asimov, Isaac
Publication:Asimov's Chronology of Science & Discovery, Updated ed.
Article Type:Reference Source
Date:Jan 1, 1994
Words:329
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