Einstein 1938: Light as Substance

Reference: Evolution of Physics

This paper presents Chapter II, section 5 from the book THE EVOLUTION OF PHYSICS by A. EINSTEIN and L. INFELD. The contents are from the original publication of this book by Simon and Schuster, New York (1942).

The paragraphs of the original material (in black) are accompanied by brief comments (in color) based on the present understanding.  Feedback on these comments is appreciated.

The heading below is linked to the original materials.


Light as Substance

Again we start with a few experimental facts. The number just quoted concerns the velocity of light in vacuo. Undisturbed, light travels with this speed through empty space. We can see through an empty glass vessel if we extract the air from it. We see planets, stars, nebulae, although the light travels from them to our eyes through empty space. The simple fact that we can see through a vessel whether or not there is air inside shows us that the presence of air matters very little. For this reason we can perform optical experiments in an ordinary room with the same effect as if there were no air.

Air has little effect on the visibility and velocity of light.

One of the simplest optical facts is that the propagation of light is rectilinear. We shall describe a primitive and naive experiment showing this. In front of a point source is placed a screen with a hole in it. A point source is a very small source of light, say, a small opening in a closed lantern. On a distant wall the hole in the screen will be represented as light on a dark background. The next drawing shows how this phenomenon is connected with the rectilinear propagation of light. All such phenomena, even the more complicated cases in which light, shadow, and half-shadows appear, can be explained by the assumption that light, in vacuo or in air, travels along straight lines.

Light travels along straight lines.

Let us take another example, a case in which light passes through matter. We have a light beam travelling through a vacuum and falling on a glass plate. What happens? If the law of rectilinear motion were still valid, the path would be that shown by the dotted line. But actually it is not. There is a break in the path, such as is shown in the drawing. What we observe here is the phenomenon known as refraction. The familiar appearance of a stick which seems to be bent in the middle if half-immersed in water is one of the many manifestations of refraction.

Light bends as it enters denser medium.

These facts are sufficient to indicate how a simple mechanical theory of light could be devised. Our aim here is to show how the ideas of substances, particles, and forces penetrated the field of optics, and how finally the old philosophical point of view broke down.

The theory here suggests itself in its simplest and most primitive form. Let us assume that all lighted bodies emit particles of light, or corpuscles, which, falling on our eyes, create the sensation of light. We are already so accustomed to introduce new substances, if necessary for a mechanical explanation, that we can do it once more without any great hesitation. These corpuscles must travel along straight lines through empty space with a known speed, bringing to our eyes messages from the bodies emitting light. All phenomena exhibiting the rectilinear propagation of light support the corpuscular theory, for just this kind of motion was prescribed for the corpuscles. The theory also explains very simply the reflection of light by mirrors as the same kind of reflection as that shown in the mechanical experiment of elastic balls thrown against a wall, as the next drawing indicates.

Light reflects like a material ball would.

The explanation of refraction is a little more difficult. Without going into details, we can see the possibility of a mechanical explanation. If corpuscles fall on the surface of glass, for example, it may be that a force is exerted on them by the particles of the matter, a force which strangely enough acts only in the immediate neighbourhood of matter. Any force acting on a moving particle changes the velocity, as we already know. If the net force on the light-corpuscles is an attraction perpendicular to the surface of the glass, the new motion will lie somewhere between the line of the original path and the perpendicular. This simple explanation seems to promise success for the corpuscular theory of light. To determine the usefulness and range of validity of the theory, however, we must investigate new and more complicated facts.



The idea of substance has been associated with matter and material properties; but substance is associated more basically with substantiality and force. Light is not a material-like substance, but it is a substance in a more basic sense.

Light seem to travel in a straight line because it has negligible inertia. The curvature of the path of travel depends on inertia (see the final comments in Chapter 1 Section 3, VECTORS). Negligible inertia means negligible curvature.

Corpuscular theory of light implies that light is made of a succession of particles following each other. It is not that simple. If there is a photon, it is not an electromagnetic particle like the atom (see the final comments in Chapter 2, Section 3, THE FIRST SERIOUS DIFFICULTY). The photon is more like a spinning thread than a spinning ball. Thus, it has two different planes of polarization.


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