Newton's corpuscular theory treated light as a stream of particles

What the theory could explain

• Reflection: Corpuscles bounce off a surface like balls bouncing off a wall. The angle of incidence equals the angle of reflection.
• Refraction: Newton argued that corpuscles speed up when entering a denser medium because they are attracted by intermolecular forces near the surface. This pulls them towards the normal, producing the observed bending.
• Dispersion: Newton proposed that different colours correspond to corpuscles of different sizes. Smaller corpuscles are refracted more than larger ones, explaining why a prism splits white light into a spectrum.
• Rectilinear propagation: Corpuscles naturally travel in straight lines unless acted upon by a force, which explains sharp shadows.

What the theory could not explain

• Diffraction: Waves bend around obstacles and spread through narrow gaps. Particles travelling in straight lines should not do this. Newton's theory had no mechanism for diffraction.
• Crucially, the corpuscular theory predicted that light travels faster in a denser medium (because the corpuscles are attracted and accelerated). This is the opposite of what was later measured experimentally.