1. Waves and Rays

2. Competing Models  Light is a stream of particles. Gassendi – 1660’sGassendi – 1660’s Newton – 1675Newton – 1675  Light doesn’t bend around corners. Particles follow straight linesParticles follow straight lines Waves bend (diffraction)Waves bend (diffraction)  Newton could explain reflection but not refraction.  Light is made of waves. Descartes – 1637 Hooke – 1660’s Huygens – 1678  Light refracts in dense media. Particles go straight Waves change direction Waves explain diffraction  Huygens describes waves moving though the ether.

3. Plane Fronts  Plane waves can be described by the direction of propagation. Vector direction  Huygens used the vectors to model the waves. Ray approximation Huygen’s principle

4. Circular Fronts  Circular waves from a point source form parallel fronts.  Lines that connect wave fronts are called rays. Perpendicular to wave fronts

5. Planes to Curves  Light waves passing through a large hole maintain straight-line rays.  Light waves passing through a small hole diffracts. Rays change directionRays change direction  The rays act like they reemerge from a point source. Limit when > dLimit when > d

6. Building a Front  Scattering from many point sources approximates a plane wave front.

7. Limited Principle  Huygens principle of rays models reflection and refraction of light. Modified for diffractionModified for diffraction Doesn’t explain size limitationDoesn’t explain size limitation  The principle fails for interference. Return to pure wave modelReturn to pure wave model next