The mysterious universe / by Sir James Jeans.

  • James Hopwood Jeans
Date:
1930
Catalogue details

Licence: In copyright

Credit: The mysterious universe / by Sir James Jeans. Source: Wellcome Collection.

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    to understand the question at issue we must retrace our steps as far back as Newton and the seventeenth century. The most obvious fact about a ray of light, at any rate to superficial observation, is its tendency to travel in a straight line; everyone is familiar with the straight edges of a sunbeam in a dusty room. As 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 pre¬ cision to it in his “corpuscular theory of light.” 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. Or its path may be bent by refraction, such as occurs when it enters water or any liquid medium; it is refraction that makes our oar look broken at the point where it enters the water, and makes the river look shallower than it proves to be when we step into it. 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 reflec¬ tion, the sine of the angle of incidence stood in a con¬ stant ratio to the sine of the angle of reflection. We
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