Particles or Waves?

The most obvious fact about a ray of light, at any rate to superficial observation, is its tendency to travel in a straight line; everyone is familiar with the straight edges of a sunbeam in a dusty room. As a rapidly-moving particle of matter also tends to travel in a straight line, the early scientists, rather naturally, thought of light as a stream of particles thrown out from a luminous source, like shot from a gun. Newton adopted this view, and added precision to it in his 'corpuscular theory of light'.

Yet it is a matter of common observation that a ray of light does not always travel in a straight line. It can be abruptly turned by reflection, such as occurs when it falls on the surface of a mirror. Or its path may be bent by refraction, such as occurs when it enters water or any liquid medium; it is refraction that makes our oar look broken at the point where it enters the water, and makes the river look shallower than it proves to be when we step into it. Even in Newton's time the laws which governed these phenomena were well known. In the case of reflection the angle at which the ray of light struck the mirror was exactly the same as that at which it came off after reflection; in other words, light bounces off a mirror like a tennis ball bouncing off a perfectly hard tennis-court. In the case of refraction, the sine of the angle of incidence stood in a constant ratio to the sine of the angle of refraction. We find Newton at pains to skew that his light-corpuscles would move in accordance with these laws, if they were subjected to certain definite forces at the surfaces of a mirror or a refracting liquid.

Newton's corpuscular theory met its doom in the fact that when a ray of light falls on the surface of water, only part of it is refracted. The remainder is reflected, and it is this latter part that produces the ordinary reflections of objects in a lake, or the ripple of moonlight on the sea. It was objected that Newton's theory failed to account for this reflection, for if light had consisted of corpuscles, the forces at the surface of the water ought to have treated all corpuscles alike; when one corpuscle was refracted all ought to be, and this left water with no power to reflect the sun, moon or stars. Newton tried to obviate this objection by attributing 'alternate fits of transmission and reflection' to the surface of the water - the corpuscle which fell on the surface at one instant was admitted, but the next instant the gates were shut, and its companion was turned away to form reflected light. This concept was strangely and strikingly anticipatory of modern quantum theory in its abandonment of the uniformity of nature and its replacement of determinism by probabilities, but it failed to carry conviction at the time.

And, in any case, the corpuscular theory was confronted by other and graver difficulties. When studied in sufficiently minute details, light is not found to travel in such absolutely straight lines as to suggest the motions of particles. A big object, such as a house or a mountain, throws a definite shadow, and so gives as good protection from the glare of the sun as it would from a shower of bullets. But a tiny object, such as a very thin wire, hair or fibre, throws no such shadow. When we hold it in front of a screen, no part of the screen remains unilluminated. In some way, the light contrives to bend round it, and, instead of a definite shadow, we see an alternation of light and comparatively dark parallel bands, known as 'interference bands'. To take another instance, a large circular hole in a screen lets through a circular patch of light. But make the hole as small as the smallest of pinholes, and the pattern thrown on a screen beyond is not a tiny circular patch of light, but a far larger pattern of concentric rings, in which light and dark rings alternate - 'diffraction rings'. All the light which is more than a pinhole's radius from the centre has in some way bent round the edge of the hole.

Newton regarded these phenomena as evidence that his 'light-corpuscles' were attracted by solid matter. He wrote:

The rays of light that are in our air, in their passage near the angles of bodies, whether transparent or opaque (such as the circular and rectangular edges of coins, or of knives, or broken pieces of stone or glass), are bent or inflected round those bodies, as if they were attracted to them; and those rays which in their passage came nearest to the bodies are the most inflected, as if they were most attracted.

Here again Newton was strangely anticipatory of present-day science, his supposed forces being closely analogous to the 'quantum forces' of the modern wave-mechanics. But they failed to give any detailed explanation of diffraction-phenomena, and so met with no favour.

In time all these and similar phenomena were adequately explained by supposing that light consists of waves, somewhat similar to those which the wind blows up on the sea, except that, instead of each wave being many yards long, many thousands of waves go to a single inch. Waves of light bend round a small obstacle in exactly the way in which waves of the sea bend round a small rock. A rocky reef miles long gives almost perfect shelter from the sea, but a small rock gives no such protection - the waves pass round it on either side, and re-unite behind it, just as waves of light re-unite behind our thin hair or fibre. In the same way sea-waves which fall on the entrance to a harbour do not travel in a straight line across the harbour but bend round the edges of the breakwater, and make the whole surface of the water in the harbour rough. The seventeenth century regarded light as a shower of particles; the eighteenth century, discovering that this was inadequate to account for small-scale phenomena such as we have just described, replaced the showers of particles by trains of waves.

(From The Mysterious Universe by Sir James Jeans.)