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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 light source.

For a description of another problem for the classical description, see the ultraviolet catastrophe.

Email link | Feedback | Contributed by: Dr. Christopher Southgate
Source: God, Humanity and the Cosmos  (T&T Clark, 1999)

Quantum Physics and Theology

Index - God, Humanity and the Cosmos, 1999 T&T Clark

The Photoelectric Effect

Related Book Topics:

The Ultraviolet Catastrophe
Collapsing Atoms
Wave-Particle Duality
The Quantum Revolution
The Schrödinger Wave Equation
The Heisenberg Uncertainty Principle
The EPR Paradox
Shaking the Foundations: The Implications of Quantum Theory
Schrödinger’s Cat and the Meaning of Quantum Theory
Does God Collapse the Wave Function?
The Hidden-Variable Theory of David Bohm
The Many-Worlds Interpretation
The Rediscovery of the Observer


Dr. Lawrence Osborn and Dr. Christopher Southgate in God, Humanity and the Cosmos. Published by T&T Clark.

See also:

Albert Einstein
Niels Bohr
Werner Heisenberg
Physics and Cosmology
The Relation of Science & Religion
A Dialogue of Scientists and Theolgians
At Home in the Quantum Universe
Books on Physics and Theology