The Photoelectric Effect
Phillip Lenard’s Experiment with Light and Electrons
Toward the beginning of the twentieth century, the problems were beginning to mount for classical physics, paving the way for the new science of quantum mechanics.
At the end of the nineteenth century, the problem of “black-body radiation,” specifically the problem known as the ultraviolet catastrophe, wherein black objects radiate energy in a manner which displayed important discrepancies with the established mathematics of classical mechanics, had become a major problem for physicists.
In 1899, Hungarian physicist Phillip Lenard formed still another experiment which challenged these laws as well. His experiment, known as the “photoelectric effect,” played a very important role in the tearing down of the classical laws, along with the black-body problem.
The Experiment
In this famous experiment, Lenard had found that an interesting phenomenon occurred when monochromatic (that is, single-colored) light was shined onto a piece of metal in a vacuum.
In such a situation, cathode rays could be produced from the metal (cathode rays are merely beams of freed electrons). This meant that the light hitting the metal had forced some electrons into being freed from the atoms. This, due to the combination of light and electrons, is called the photoelectric effect.
Lenard performed further experiments wherein he attempted to increase the intensity of the light (that is, he turned up the light’s brightness), measuring the quantity and the velocity of the electron output as he did so.
What he found was surprising. One might expect that as the intensity of the light increased, the waves of light should be striking the surface with more intensity, thus increasing the intensity (speed) at which the electrons themselves jumped from the metal.
Instead, what was occurring was that as the intensity of the light increased, a greater number of electrons were being freed, but they appeared to be moving at exactly the same speed as when the light was dim.
Brighter light did not mean more energetic electrons, but a greater quantity of them.
Further experiments showed that it was only when the color of the light being used was changed that the intensity of the electrons were found to be effected. Shorter wavelengths produced more energetic electrons than longer wavelengths.
A Summary of the Problem
Clearly, this phenomenon, like the black-body problem, did not quite fit in with classical physics – if light behaved as a continuous wave, as had been shown by the experiments of Thomas Young and others in the beginning of the nineteenth century, an increase in the intensity of a light should increase the intensity of the waves, thus increasing the intensity of the electrons.
Not so.
This little problem, when combined with the problem of Black-body radiation, led to a two-pronged attack on the problem within the first five years of the new century. The first – the solving of the Black-body problem during the final days of 1900 by Max Planck, and the second in 1905 by Albert Einstein (one of the four important scientific discoveries he made in that, his “miracle year.”
References:
Einstein, A. (1905). On a Heuristic Point of View about the Creation and Conversion of Light. Annalen der Physik .
Gribbin, J. (1994). In Search of Schrodinger's Cat: Quantum Physics and Reality. New York, NY: Bantam Books.
Herbert, N. (1985). Quantum Reality. Garden City, NY: Anchor Press/Doubleday.
