The Quantization of Energy 173
There are three aspects of this photoelectric effect, however, which
cannot be understood in terms of classical physics. The first is the
threshold effect of frequency. Unless the frequency of the light is above
the threshold frequency, no photoelectrons are ejected from the metal, no
matter how high the intensity of the light. There is no reason why this
effect should depend upon the frequency, as the experimental data
seemed to indicate. The second peculiar aspect of the photoelectric effect
is that photoelectrons appear the instant the metal is irradiated by light.
The electrons are ejected within 10-8 seconds of the light striking the
surface of the metal, independent of the intensity of the light, as long as
the frequency is high enough. This contradicts the classical picture of the
electron absorbing energy from the light wave, since it is easy to show
that 10-8 sec does not allow sufficient time for the electron to absorb
enough energy from a wave to overcome the electromagnetic forces,
which hold it captive within the metal. The third peculiar aspect of the
photoelectric effect was pointed out by Leonard in 1902. He observed
that, when the intensity of light was increased by moving the source
closer to the metal, for example, the energy of the electrons ejected from
the metal did not increase but rather more electrons appeared. If one
increases the frequency of the light projected on the metal, however, then
the energy of the ejected electrons will increase.
Einstein was able to explain all three of these mysterious effects in his
brilliant paper of 1905, on the photoelectric effect for which he was
awarded the Nobel Prize in 1921 (Einstein never received a Nobel Prize
for his work on relativity). Einstein expanded Planck’s quantum
hypothesis by assuming that, not only the light induced by thermal
radiation is quantized, but, that all electromagnetic radiation comes in
bundles of energy or photons. The energy of an individual photon
composing a light ray equals hf where h is Planck’s constant and f is the
frequency of the radiation. According to Einstein’s hypothesis, at certain
times, light behaves like a beam of particles where each particle is a
photon. But, how does this explain, one might ask, the mysterious
aspects of the photoelectric effect? Well, in order for an electron to
escape the metal, it must have a sufficient amount of energy to overcome
the normal electromagnetic forces, which keep it captive inside the
metal, i.e. it must have enough energy to overcome the binding energy. If
the frequency of the light is too small, then the energy of its quanta or
photons is less than the binding energy and the photon cannot transfer
enough energy to the electron for it to overcome the forces keeping it