Photoelectric effect in conflict with beliefs of the time

A series of experiments conducted in the late 19 th century revealed that electrons are emitted from a metal surface when light of sufficiently high frequency falls upon it. Ultra violet light is required for most metals, though photoelectric emission occurs for metals like sodium, potassium and caesium, using visible light. The metal plate, which releases the electrons, is called the emitter. If a second conductor called a collector, is placed near the emitter and maintained at a positive potential with respect to it, a current will flow. Even when no potential difference exists between the collector and emitter, a small current flows. In fact, a small retarding voltage is required to stop the most energetic electrons from getting across. The velocity of the most rapidly moving electrons can be found by measuring the “stopping potential” required to reduce the current to zero.

Superficially this is a simple and logical effect, since light is a transmission of energy and the energy it imparts to the electrons allows them to escape. However two features of the effect puzzled the early observers of it:

Maxwell’s theory of electromagnetic radiation which was universally accepted at the time, considered light to be a continuous wave motion in which the energy carried by the wave was distributed evenly over the wave front. In this event, an equal amount of energy would be given to each of the millions of atoms on the surface of the plate and it would need to be exposed to the radiation for some hours before any one electron would have acquired enough energy to escape. However emission was spontaneous.

Secondly, under Maxwell’s Theory, the more intense a beam of light, the more energy it carried, and therefore the electrons released by an intense beam of light ought to have greater kinetic energy. This was found not to be so. Increasing the intensity released more electrons, bur didn’t increase their energy. What was found to affect their energy was the frequency of the incident light. The higher the frequency of the light, the more energetic were the electrons emitted.

In fact it was found that with a given metal as emitter, the frequency of the incident light must be higher than a certain critical value called the threshold frequency. This value is different for different metals.

The difficulties posed by the photoelectric effect including spontaneous emission and the existence of a threshold frequency were overcome in 1905 when Einstein proposed a new interpretation of the effect.

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Theory
Einstein's solution
Photons

Glossary