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THE PHENOMENON
When certain kinds of light are shone upon metals, electrons are liberated. Each of the liberated electrons leaves the metal with a kinetic energy anywhere from near zero up to a well-defined maximum. When the color, and hence the frequency, of the light is changed, the maximum energy of the liberated electrons is greater or smaller, depending upon the frequency change; itís a directly proportional relationship. If the frequency drops below a certain number, no electrons are liberated, no matter how intense the light.
Summary of Photoelectric Effect
CHANGE IN LIGHT EFFECT ON ELECTRONS Increased frequency Increased maximum electron energy Increased intensity Greater number of electrons freed Frequency lowered below threshold Absolutely no electrons freed, ever
WHY THE WAVE EXPLANATION DOES NOT WORK
From a "wave" perspective, intensity is associated with the amplitude of the wave. By increasing the intensity, one would expect the wave to be more energetic, to "hit harder." After all, a big wave on the beach carries away more sand than a tiny wave. But, in the case of, say, red light with certain metals, no electrons are liberated NO MATTER HOW LARGE A WAVE IS EMITTED. In addition, for the same intensity of two different light sources, one green and the other blue, you will get the same number of liberated electrons but the "green" electrons will be less energetic (slower) on average than the "blue" electrons. The energy change of the electrons is due to changes in light frequency, not intensity.
WHY THE PARTICLE EXPLANATION DOES WORK
From a "particle" perspective, an increased intensity corresponds to an increased number of photons (quantized particles of energy, as Planck had suggested). If there are more photons hitting the metal, there can be more photon-electron "collisions," and more liberated electrons. An increased frequency is harder to state in terms of particles. Practically, a change in frequency means a change in color; red light is the visible light with lowest frequency, violet-blue has the highest frequency in the visible range. Planck had already determined that the energy came in discrete quanta according to the relationship:
E = h * f
where h = 6.6 x 10 ó34 joules-sec (Planckís constant, naturally).
So, change the color, change the frequency, and change the energy of the particle. This inspired Einstein to imagine the photoelectric effect as a collision between two particles ó the then newly-named "photon" and the then nearly-decade-old-named electron. The Planck energy stated above represents the maximum amount of kinetic energy the electron might have upon liberation; some energy initially absorbed by the electron can be lost as the electron makes it way through the metal which previously bound it. Those electrons closest to the surface will have close to the maximum amount of energy, namely hf.
Therefore, if you release a small number of photons, you will get a small number of liberated electrons, each with an energy associated with that frequency of light. If the frequency of light is too low, the photons will arrive with an energy insufficient to knock loose an electron and, NO MATTER HOW MANY PHOTONS ARE RELEASED, no electrons will be liberated.