Reference: Evolution of Physics
This paper presents Chapter II, section 6 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.
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The Riddle of Colour
It was again Newton’s genius which explained for the first time the wealth of colour in the world. Here is a description of one of Newton’s experiments in his own words:
In the year 1666 (at which time I applied myself to the grinding of optick glasses of other figures than spherical) I procured me a triangular glass prism, to try therewith the celebrated phenomena of colours. And in order thereto, having darkened my chamber, and made a small hole in my window-shuts, to let in a convenient quantity of the sun’s light, I placed my prism at its entrance, that it might thereby be refracted to the opposite wall. It was at first a very pleasing divertisement, to view the vivid and intense colours produced thereby.
The light from the sun is “white”. After passing through a prism it shows all the colours which exist in the visible world. Nature herself reproduces the same result in the beautiful colour scheme of the rainbow. Attempts to explain this phenomenon are very old. The Biblical story that a rainbow is God’s signature to a covenant with man is, in a sense, a “theory”. But it does not satisfactorily explain why the rainbow is repeated from time to time, and why always in connection with rain. The whole puzzle of colour was first scientifically attacked and the solution pointed out in the great work of Newton.
Newton used a triangular glass prism to produce from sunlight all the colors which exist in the visible world. This phenomenon is similar to the rainbow which is produced when sunlight passes through the water droplets of the rain.
One edge of the rainbow is always red and the other violet. Between them all other colours are arranged. Here is Newton’s explanation of this phenomenon: every colour is already present in white light. They all traverse interplanetary space and the atmosphere in unison and give the effect of white light. White light is, so to speak, a mixture of corpuscles of different kinds, belonging to different colours. In the case of Newton’s experiment the prism separates them in space. According to the mechanical theory, refraction is due to forces acting on the particles of light and originating from the particles of glass. These forces are different for corpuscles belonging to different colours, being strongest for the violet and weakest for the red. Each of the colours will therefore be refracted along a different path and be separated from the others when the light leaves the prism. In the case of a rainbow, drops of water play the role of the prism.
According to Newton every color is already present in white light; the prism separates them in space. According to the mechanical theory, refraction is due to forces acting on the particles of light and originating from the particles of glass.
The substance theory of light is now more complicated than before. We have not one light substance but many, each belonging to a different colour. If, however, there is some truth in the theory, its consequences must agree with observation.
The series of colours in the white light of the sun, as revealed by Newton’s experiment, is called the spectrum of the sun, or more precisely, its visible spectrum. The decomposition of white light into its components, as described here, is called the dispersion of light. The separated colours of the spectrum could be mixed together again by a second prism properly adjusted, unless the explanation given is wrong. The process should be just the reverse of the previous one. We should obtain white light from the previously separated colours. Newton showed by experiment that it is indeed possible to obtain white light from its spectrum and the spectrum from white light in this simple way as many times as one pleases. These experiments formed a strong support for the theory in which corpuscles belonging to each colour behave as unchangeable substances. Newton wrote thus:
. . .which colours are not new generated, but only made apparent by being parted; for if they be again entirely mixt and blended together, they will again compose that colour, which they did before separation. And for the same reason, transmutations made by the convening of divers colours are not real; for when the difform rays are again severed, they will exhibit the very same colours which they did before they entered the composition; as you see blue and yellow powders, when finely mixed, appear to the naked eye, green, and yet the colours of the component corpuscles are not thereby really transmuted, but only blended. For when viewed with a good microscope they still appear blue and yellow interspersedly.
Newton showed by experiment that it is indeed possible to obtain white light from its spectrum and the spectrum from white light in this simple way as many times as one pleases. The colours of the component corpuscles are not thereby really transmuted, but only blended. We have not one light substance but many, each belonging to a different color.
Suppose that we have isolated a very narrow strip of the spectrum. This means that of all the many colours we allow only one to pass through the slit, the others being stopped by a screen. The beam which comes through will consist of homogeneous light, that is, light which cannot be split into further components. This is a consequence of the theory and can be easily confirmed by experiment. In no way can such a beam of single colour be divided further. There are simple means of obtaining sources of homogeneous light. For example, sodium, when incandescent, emits homogeneous yellow light. It is very often convenient to perform certain optical experiments with homogeneous light, since, as we can well understand, the result will be much simpler.
The consequence of the theory is the possibility of homogenous light, which cannot be split into further components. The existence of such light can easily be demonstrated by experiments.
Let us imagine that suddenly a very strange thing happens: our sun begins to emit only homogeneous light of some definite colour, say yellow. The great variety of colours on the earth would immediately vanish. Everything would be either yellow or black! This prediction is a consequence of the substance theory of light, for new colours cannot be created. Its validity can be confirmed by experiment: in a room where the only source of light is incandescent sodium everything is either yellow or black. The wealth of colour in the world reflects the variety of colour of which white light is composed.
The substance theory of light predicts that if sun emitted only homogeneous light of some definite color, such as yellow; the great variety of colors on the earth would immediately vanish. This can be demonstrated experimentally.
The substance theory of light seems to work splendidly in all these cases, although the necessity for introducing as many substances as colours may make us somewhat uneasy. The assumption that all the corpuscles of light have exactly the same velocity in empty space also seems very artificial.
According to the substance theory of light it appears that there are as many substances as there are colors; and they all may not have exactly the same velocity in empty space.
It is imaginable that another set of suppositions, a theory of entirely different character, would work just as well and give all the required explanations. Indeed, we shall soon witness the rise of another theory, based on entirely different concepts, yet explaining the same domain of optical phenomena. Before formulating the underlying assumptions of this new theory, however, we must answer a question in no way connected with these optical considerations. We must go back to mechanics and ask: WHAT IS A WAVE?
The substance theory of light seems to work splendidly, but there can be an entirely different theory that can explain the same phenomena and more.
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Final Comment
Light is also a weightless substance like heat, electricity and magnetism. But different colors make this substance theory of light very complex. There could be a greater simplicity underneath this complexity of different weightless light substances.
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