Part II light: particles or waves.

The most obvious fact about a ray of light, superficially observed is its tendency to travel in a straight line. Because 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".

Figure 1 – Light turned by reflection at M Figure 2 – Light turned by refraction S

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 (Fig. 1). Or its path may be bent by refraction, such as occurs when it enters water or an liquid medium; it is refraction that makes an oar look bent at the point where it enters the water, and makes the river look shallower than it proves to be when we step into it (Fig.2). 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 (Fig. 4). Newton took great care to try to prove that the light-corpuscles would move in accordance with these laws, if they were subjected to certain definite forces at the surface of a mirror or a refracting liquid.

Figure 3 – Reflection and refraction at N Figure 4 –sin ANR = AR = refractive sin BNS

BS index

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 (Fig. 3). The objection to Newton's theory was that it 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. In other words, when one corpuscle was refracted, all ought to be, and this left water with no power to reflect the sun, moon or stars1. Newton tried to get around 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 light2. The concept was surprisingly close to modern quantum theory in its abandonment of the uniformity of nature and its replacement of determinism by probabilities, but this explanation was not accepted at the time.

The corpuscular theory also faced other, more serious difficulties. When studied in sufficiently minute details, it is found that light does not 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 good protection from the glare of the sun. But a tiny object, such as a very thin wire, hair or fibre, throws no such shadow. When it is held in front of a screen, no part of the screen remains unilluminated. In some way, the light manages 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 if the hole is small as the smallest of pinholes, then he 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; these are called " 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 on 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 were closely analogous to the quantum forces of modern wave-mechanics. But they failed to give a detailed explanation of diffraction phenomena, and were therefore not accepted.

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 metres long, many thousands of waves are in a single inch. Waves of light bend round a small obstacle in exactly the same way in which waves of the sea bend around a small rock A rocky reef miles long gives almost perfect shelter from the sea; 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 a thin fibre or hair. Similarly, 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.