1. The Physics of Light
And God said, Let there be light: and there was light.
And God saw the light, that it was good:

2. Light - Introduction to Physical Optics
Chapter 22: Reflection, Refraction, Snell’s Law, Dispersion and the Wave-Particle Debate
Chapter 24: Interference, Diffraction and Polarization
Chapter 25: Some applications to Biology and technology

3. What is Light? Basic Properties…
rectilinear (straight-line) propagation
finite speed ( km/s)
carries energy (E = hf)
reflects and refracts
can be broken into a spectrum

4. The Wave-Particle Debate
Newton
light consists of small “massy” particles or corpuscles that travel in straight lines, are subject to forces as one would expect of particles but also have additional vibratory properties that can be used to explain dispersion, color produced by oil slicks and so on.
Huygens
light propagates as a wave disturbance through the ether - an unseen, elastic medium pervading all of space. Light will add, cancel and share properties common to all waves.

5. Huygen’s Principle
... every point on a primary wavefront serves as the source of spherical secondary wavelets propagating in the forward direction such that the primary wavefront at some later time is the envelope of these wavelets. Further, the wavelets advance with a speed and frequency equal to the primary wave at each point in space. simulate this...

6. Explaining Reflection and Refraction
Law of Reflection….
Newton…
conservation of momentum and simple application of physics of forces
Huygens…
wave superposition and interference (applet)
normal qi qr

7. Law of Refraction….Snell’s Law...
Newton
attractive forces
Huygens
wave interference qi qr

8. A curious twist...
Both models correctly explain (“predict”) Snell’s Law … but … in so doing they make opposite ancillary predictions:
Newton - light travels faster through dense medium
Huygens - light travels slower through dense medium

9. General form for Snell’s Law and Total Internal Reflection
In general… so ... n1 n2 n3

10. Total Internal Reflection? n2
If a ray hits the surface at an angle steeper than the critical angle it will be reflected back into the original medium