Theories of Light

• What did Newton propose light was made of?

  Small particle-like bodies called corpuscles
• What was one prediction of Newton's corpuscular theory?

  Objects emitting light were losing mass slowly
• Which phenomena could Newton's theory explain?

  Reflection, refraction, and dispersion
• What phenomenon could Newton's theory not explain?

  Diffraction
• How did corpuscular theory explain reflection?

  Corpuscles hit the surface and repelled
• What law does the corpuscular theory follow during reflection?

  Newton's third law
• How did corpuscular theory explain refraction?

  Assumed attraction between light and matter
• What happens to corpuscles at a boundary with a denser medium?

  Resultant force acts perpendicular to the boundary

  
• What is a consequence of light traveling in a denser medium?

  Light travels faster in a more dense medium
• How does the boundary force affect the velocity of corpuscles?

  Increases the vertical component of velocity

  
• Who proposed the wave theory of light?

  Huygens
• What did Huygens suggest about the medium for light?

  The Universe was filled with luminiferous aether
• How did Huygens describe the propagation of light?

  Light travelled in wavefronts
• What happens at any point of a wavefront according to Huygens?

  It acts as a secondary point source

  
• How does Huygens' theory explain reflection?

  New wavelets form from the point of reflection
• How does Huygens' theory explain refraction?

  Light travels slower in a denser medium
• What are the similarities and differences between corpuscular and wave theories of light?

  Similarities:

  • Both explain reflection
  • Both explain refraction
  • Both explain dispersion

  
  

  Differences:

  • Corpuscular theory: light as particles with mass
  • Wave theory: light as waves in a massless medium
  • Corpuscular theory: light faster in denser media
  • Wave theory: light slower in denser media
  • Corpuscular theory: no explanation for diffraction or interference
• Why was Newton's theory more accepted than Huygens' theory?

  Newton was widely respected for his work
• What limited the acceptance of Huygens' theory during Newton's time?

  No way to measure the speed of light
• What changed the acceptance of Newton's theory after 150 years?

  Observation of diffraction patterns of light
• What are key tips for exam questions on this topic?

  • Focus on explanations of reflection and refraction
  • Remember main differences between theories
  • Understand why Huygens' theory was less popular