Number of words: 316

In atomic physics, many of the paradoxical situations are connected with the dual nature of light or – more generally of electromagnetic radiation. On the one hand, it is clear that this radiation must consist of waves because it produces the well-known interference phenomena associated with waves: when there are two sources of light, the intensity of the light to be found at some other place will not necessarily be just the sum of that which comes from the two sources, but may be more or less. This can easily be explained by the interference of the waves emanating from the two sources: in those places where two crests coincide we shall have more light than the sum of the two; where a crest and a trough coincide we shall have less. The precise amount of interference can easily be calculated. 

Interference phenomena of this kind can be observed whenever one deals with electromagnetic radiation, and force us to conclude that this radiation consists of waves. On the other hand, electromagnetic radiation also produces the so-called photoelectric effect: when ultraviolet light is shone on the surface of some metals it can ‘kick out’ electrons from the surface of the metal, and therefore it must consist of moving particles. A similar situation occurs in the ‘scattering’ experiments of X-rays. These experiments can only be interpreted correctly if they are described as collisions of ‘light particles’ with electrons. And yet, they show the interference patterns characteristic of waves. The question which puzzled physicists so much in the early stages of atomic theory was how electromagnetic radiation could simultaneously consist of particles (i.e. of entities confined to a very small volume) and of waves, which are spread out over a large area of space. Neither language nor imagination could deal with this kind of reality very well. 

Excerpted from Page 47 of ‘The Tao of Physics’ by Fritjof Capra