Einstein's aberration experiment

Abstract

In the Sitzungsberichte of the Berlin Academy of December 8 last, which has recently come to hand, Einstein describes an ingenious arrangement which he suggests might serve to decide between the classical theory of light and the theory in which light is regarded as made up of single quanta of energy emitted discontinuously from luminous atoms. Fig. 1 (reproduced from the paper) illustrates the proposed experiment. K is a stream of canal rays, L1 is a focussing lens, S is a screen containing a slit which serves to isolate a definite pencil of light, and the lens L2 renders the emergent beam parallel. The emergent pencil is observed through a telescope focussed for infinity, so that the image of the slit in the screen S would be seen sharply focussed in the field of view. Since the atoms in the canal rays emitting light are in motion, the Doppler effect comes into evidence, and the rays proceeding at any instant from individual luminous atoms in different directions should, according to the wave-theory of light, be of different frequencies. Einstein suggests that the rays passing through the slit S and incident on the upper and lower parts of the lens L2 should consequently be of different frequencies. If, therefore, a layer of a dispersing medium such as carbon disulphide be placed between the lens L2 and the observing telescope, the different rays would travel through it with different velocities. Hence the wave-front should suffer an aberration and the image of the slit seen in the focal plane should shift through an extent proportionate to the thickness of the dispersing layer introduced. Einstein conceives that according to the quantum theory of light, on the other hand, such displacement should not occur, and he believes that the proposed arrangement furnishes an experimentum crucis to decide between the rival theories.