Particle theory vs. Wave theoryParticles have mass (inertia) and respond toexternal forces (gravity / acceleration).Particles have momentum (mass x velocity).The “Corpuscular Theory of Light” (Newton - 1670)Light “Corpuscles” have mass and travel at extremelyhigh speeds in straight lines.Blocked by large objects (well-defined shadows).Huygens Principle (1678)The “Wavelet envelope” model (each point on a wavefront actsas a source for the next wavefront).Plane waves generate plane waves, circular waves generate circular waves.Light composed of longitudinal waves (like sound).