Theories of Light (AQA A Level Physics)

Newton’s Corpuscular Theory of Light

• Newton proposed that light was made of small particle-like bodies called corpuscles, emitted by luminous objects

  • One prediction of this theory was that objects emitting light were losing mass slowly

  • This theory was able to explain reflection, refraction and dispersion, but not diffraction

• To explain reflection:

  • The corpuscles simply hit the reflective surface and experienced an equal and opposite repulsive force from the surface, following Newton's third law

  • This is because corpuscular theory treated corpuscles like solid, elastic spheres

• To explain refraction:

  • Corpuscular theory assumed there was a force of attraction between light and matter

  • In a single medium, such as air, the force supposedly acted on all sides so there was no resultant force

  • But at a boundary between air and a denser medium, Newton said there was a resultant force on the corpuscles acting perpendicular to the boundary, because there was more matter in the new medium

  • A consequence of this was that light travelled faster in a more dense medium

A diagram showing Newton's corpuscular explanation for refraction

The boundary force, arising from an attraction to a greater amount of matter, increases the vertical component of velocity, which changes the angle of the corpuscle's path - this means the magnitude of the velocity is greater in water

Comparing Corpuscular & Wave Theories

Huygens' Wave Theory of Light

• Huygens was a Dutch scientist who proposed that light was a wave

  • All other known waves travelled through a medium so he suggested the Universe was filled with a massless medium known as the "luminiferous aether" (more on this in Special Relativity)

• In his theory, light travelled in wavefronts

  • These wavefronts were emitted from a point source

  • Any point of the wavefront then acted as a secondary point source, from which wavelets could propagate

  • These wavelets joined together to form a new wavefront, and so on

Diagram showing the propagation of wavefronts 

Any point on a wavefront can act as a secondary point source for wavelets - only a few are shown here

• To explain reflection:

  • When a wavefront hits a reflective surface, the point of reflection becomes a secondary point source for new wavelets

  • Different parts of the wavefront hit the reflective surface at different times, so the new wavefront forms in a new direction

• To explain refraction:

  • This theory, in contrast to Newton's corpuscular theory, relied on light travelling slower in a denser medium

  • Again, different parts of the wavefront hit the boundary to a new medium at different times

  • The part of the wavefront which first reaches the boundary slows before the rest of the wavefront, causing it to change direction 

Comparing the two Theories

• Similarities between the two theories:

  • Both explained reflection

  • Both explained refraction

  • Both could explain dispersion

• Differences between the two theories:

  • Corpuscular theory said light was composed of particles with mass, while wave theory said it was a wave travelling through a massless medium

  • Corpuscular theory claimed light travelled faster in denser media, whereas wave theory claimed light travelled slower in denser media

  • Corpuscular theory had no explanation for diffraction or interference, however, these were common properties of waves

Why was Newton's theory more accepted than Huygens'?

• Both theories explained the phenomena of light, but both also had flaws

• Newton was already widely respected thanks to his work on motion and gravity

• There was no way of measuring the speed of light or observing diffraction of light at the time, so corpuscular theory was the accepted theory of light for 150 years

  • This changed when diffraction patterns of light were observed that contradicted this theory