On the direction of motion of an electron ejected from an atom by ultra-violet light

Abstract

When light is absorbed by matter it exerts, according to the electro­ magnetic theory of light, a pressure upon the matter, or gives a momentum to it, in the direction of propagation of the light. The existence of this pressure has been proved by a number of observers. When ultra-violet light is absorbed by matter we know that a part at least of the absorbed energy is expended in liberating cathode rays of small velocity. The momentum corresponding to the absorption of energy in this case thus probably appears in part in the form of moving electrons. We would, therefore, expect these cathode rays to have a component of velocity in the direction of propagation of the light. The results that have been obtained in experiments on the secondary cathode radiation from substances exposed to γ - and X-rays also suggest that possibly such an effect exists. Thus the larger amount of secondary β radiation given off from the side of a radiating plate where the stream of exciting γ -rays emerges than where it enters has been explained by Prof. Bragg by supposing that the secondary β -rays are projected in the direction of propagation of the γ -rays. Cooksey showed that a similar effect exists with X-rays. It appeared a promising experiment therefore to look for such an effect in the case of ultra-violet light, and the writer accordingly carried out a set of measurements to test this point.