March 11, 2006

The Particle Nature of Light

Albert Einstein, On a Heuristic Point of View Concerning the Production and Transformation of Light, 1905

In 1905, Einstein didn't look anything like the later popular images of an older man with wildly unkempt white hair. When he published this paper, which later won him the Nobel Prize, he was a patent clerk, and looked it:

In this paper, Einstein points out that there is a mathematical similarity between the way black-box radiation behaves and the way a gas of particles behaves, and takes the crucial step of suggesting that light is, in fact, particulate. This goes one step further than Planck, who only showed that atoms absorb and admit light energy in discrete quanta.

Along the way, Einstein derives h, Planck's constant, in a new way, confirming that the energy of a quantum of light is h times the frequency of the light. After this theoretical triumph, Einstein uses his model of light to explain some puzzling experimental observations:

In 1902, Philip Lenard observed the photoelectric effect: when you shine ultraviolet light on an object, it emits electrons. Lenard noted that the kinetic energy of the escaping electrons did not vary with the intensity of the light, but instead increased with the frequency of the light, which was unexpected and difficult to explain.

Einstein proposed that an electron absorbs one quantum of light, and if this imparts enough energy to allow it to escape its atom, it does, with the kinetic energy of that quantum minus the energy needed to escape from the atom. This prediction was confirmed a decade later. More to the point, increasing the intensity of light increases the number of incoming quanta, and thus the number of escaping electrons, while increasing the frequency increases the energy of the quanta and electrons. This effect is simple, definitive evidence that light energy can only be transferred to electrons in discreet chunks of known, fixed size.

Einstein's genius was partly in the simplicity of his arguments. His conclusions are, on occasion, mind-boggling, but the arguments that lead to those conclusions are often easy to understand. (As with Special Relativity.)

Interestingly, the idea that light is made of particles may be an illusion created by the fact that electrons can only absorb and emit light in discreet amounts. I think this is why Einstein gave his paper the title On a Heuristic Point of View Concerning the Production and Transformation of Light. Are there any experiments that show that a quantum of light energy is concentrated into a point in space? Can there be, if electrons can only absorb or admit light in quanta?


At March 13, 2006, Blogger Fraxas said...

Great article.

You've come up against an observability problem there, I think. The only experiment I can think of that *might* let you determine the locality of a quantum of light would be one that relied on relativistic space distortion measurement, since that's all I can think of that wouldn't interfere (literally!) with the quantum being measured.


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