Newton's geometric theory of light also held that we see because particles of light emanating from objects (or bouncing off of them) enter our eyes. He viewed a succession of light particles emanating from a given point on an object as a light ray. In Newton's view, light particles travel in straight lines and the various colors of light correspond to light particles of differing "mass" or size. His view of light had the light particles obeying the same laws of motion as he had developed for things like rocks or planets.

But, the particle theory of light ran into trouble with the phenomena of light interference and diffraction. A simple example can be found in the fuzziness around the edges of a shadow of an opaque object like a rock. If light always travels in straight lines, then the outline of a shadow should be every bit as sharp and clear as the edge of the rock. Indeed, the outlines of shadows do appear to be sharp. However, closer examination of the shadow shows a measure of blur in the edge. The shadow should appear completely black, as the light illuminating the rock is being blocked by it so that no light from the projected shadow can be reflected back to your eyes. (This region of shadow is called the umbra.) But near the edge, the shadow is noticeably lighter in shade of gray.

If the bright source illuminating the rock casting the shadow is noticeably broad, then an intermediate shadow region is formed (called the penumbra). The penumbra is clearly shadow, but it is not as dark as full shade. The reason for it comes simply from the geometry of straight lines for light rays originating at different positions on the source. If the width of the light source, as viewed from the rock's position, can be reduced, the penumbra becomes narrower as well. But, there is still a fuzziness around the edges of a shadow that cannot be explained by penumbra.