Photoelectric Effect - Einstein's Explanation

Prior to Albert Einstein, the field of Physics was in turmoil regarding the Nature of Light. For centuries, half of all scientists believed that it was a WAVE phenomenon, while the other half of scientists insisted that it was a PARTICLE phenomenon. Both sides had long known how to perform experiments to PROVE that their view was correct. The problem was that everyone realized that Light could not possibly be BOTH, for a very specific reason. Any PARTICLE phenomenon has a smallest limit which it could be, after repeatedly chopping it in half, after which it could no longer exist as that particle, but necessarily as something that was smaller. In contrast, a WAVE phenomenon can forever be chopped in half, always remaining the same type of wave.

So for around two centuries, ferocious and vicious arguments occurred between scientists of the two different attitudes. Each was confident about doing experiments to support and defeend their view, but no one seemed to be able to do any experiments that might show the other side to be wrong!

Around 1887, experiments involving the Photoelectric Effect were especially troublesome to scientists. Assuming that light was a WAVE phenomenon, everyone assumed that BRIGHTER light must necessarily cause greater electric effect, essentially in direct proportion to the intensity of the light, but all experiments showed that was not true.

Experiments instead seemed to show that the photoelectric effect was directly proportional to the COLOR of the light, that is, the wavelength. It did not seem to matter how bright a RED light source was, that no photoelectric effect occurred. In contrast, even very weak or dim VIOLET light created the photoelectric effect!

Einstein determined that light MUST sometimes act as though it was a PARTICLE phenomenon (which is what got Einstein his one and only Nobel Prize). Einstein concluded that when light acted as if it is a particle, what he called a PHOTON, the energy contained in that photon depended entirely on the FREQUENCY or WAVELENGTH of the color of the light. He determined that a RED photon did not contain enough energy to kick any energy from the metal atoms in the photoelectric effect, so the effect did not then occur.

Einstein concluded that LIGHT has the capability of a DUAL nature. Depending on what experiment was performed, light might act as though it is a wave phenomenon OR a particle phenomenon.

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For the following hundred years, all Physicists have fully accepted Einstein's explanation of the Photoelectric Effect and also the dual nature of light. Another later consequence of that is that the concept of Quantum Dynamics was postulated and fully accepted.


This presentation was first placed on the Internet in March 2014.

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago