Newton’s Corpuscular Theory of Light

Newton proposed that a luminous source sends out a stream of tiny particles called corpuscles. In this model, light travels in straight lines. An opaque object blocks the corpuscles travelling towards it, leaving a region behind it that receives no light. This is consistent with the formation of a sharp shadow.

Newton explained reflection as corpuscles rebounding from a surface. He explained refraction as a change in the motion of corpuscles when they cross a boundary between two media; their direction of travel can change at the boundary.

Comparison with Huygens’ wave theory

Huygens proposed instead that light travels as a wave spreading through space. In this model, reflection and refraction result from changes in the propagation of wavefronts at a boundary between media. A wave can also spread around an obstacle, unlike a stream of independent corpuscles travelling in straight lines.

Why the corpuscular theory was preferred

Newton’s theory was preferred for a long time partly because of his scientific authority and reputation. Its apparent agreement with straight-line travel and sharp shadows was also persuasive. The wave theory raised a question: if light is a wave, why does it not spread noticeably around obstacles? Wave spreading can be too small to notice, so a sharp-looking shadow does not by itself rule out a wave model.