||The nature of light has long been of absorbing interest to humankind. Newton, in the mid-17th century, was the first person to apply serious thought to the problem, concluding that light was made up of many tiny particles. However, Huygens later proved that light is far more easily explained as a wave. The wave-like behaviour of light is exhibited in phenomena such as diffraction and refraction.
To illustrate the difference between wave-like and particle-like behaviour, we may consider the behaviour of water waves. If a series of waves hits a barrier with a small gap in it, they will spread out on the far side of the gap in a series of circular ripples (a process known as diffraction). If, however, a beam of particles passes through a small gap, they will not spread out, but continue in a straight line.
If a beam of light is shone upon a small hole, it diffracts exactly like water waves. This behaviour, both in light and in water waves, is seen when the gap is of a similar size to the wavelength. Thus light, which has a very small wavelength, only exhibits this behaviour when the hole is very small.
This would appear to show that light is a wave. But other evidence says otherwise. If the energy of a light beam is detected, it is not deposited smoothly, as a wave would be. Instead, it is detected in separate quanta, the size of which are determined by the colour or frequency of the light. It is, in fact, impossible to detect light other than in discrete packages: it is quantized. The packages are called photons.
This would appear to suggest that light is composed of particles. Scientists may conclude, from these two modes of behaviour, that light travels as a wave (diffraction), but interacts as a particle (quantized energy).
Current theories lead to various ways of resolving this problem. One way of thinking about it is to say that light is neither purely a wave nor a particle. We may model it with waves for some situations, and with particles for others, but it is a thing different from either, possessing the properties of both.
A different formulation of the behaviour of light has been given by Feynman. He describes mathematically the whole range of properties of light by assuming that it is a particle with phase properties, in his theory of quantum electrodynamics. JJ
See also de Broglie waves; optics, refraction and reflection; quantization.