3.12.2.1
Newton's corpuscular theory treated light as a stream of particles
Turning Points in Physics | AQA A-Level Physics
Key Definition
Corpuscular theory: Newton's model of light, which proposed that light consists of tiny particles (corpuscles) emitted from a source and travelling in straight lines at high speed.
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.
Common Mistake
A common exam error is saying the corpuscular theory predicted light would be "slower in glass." It predicted the opposite: faster in a denser medium. Huygens' wave theory was the one that correctly predicted light would be slower in glass. This distinction is a frequent exam question.