Theories of Light (AQA A Level Physics)

• 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

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