The Photoelectric Effect
At the beginning of the 20th century, physicists
turned their attention to the ability of light to eject electrons from metal.
This could be explained as light imparting energy to electrons which then
effectively evaporate from the surface of the metal. The classical analogy
with the evaporation of water suggests that some degree of evaporation should
occur regardless of the frequency of the light, provided it is sufficiently
intense. In reality, there is a clear threshold frequency, which varies from
metal to metal, below which the effect will not occur.
It was Einstein who, in 1905, explained this anomaly by
assuming that the energy imparted by the light is packaged (quantised) in a
manner that is related to the frequency of the light rather than spread evenly
over the wavefront. This effect is more akin to light behaving as packets of
energy - indeed as particles - than as waves (see wave-particle duality).
Furthermore, Einstein assumed that the way in which electrons absorbed that
energy is also quantised - so that they can only acquire the energy necessary to
escape if the light is of sufficiently high frequency. Light of frequencies
lower than this threshold has no effect regardless of the intensity of the
For a description of another problem for the classical
description, see the ultraviolet catastrophe.
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Source: God, Humanity and the
Cosmos (T&T Clark, 1999)