PH 103 Dr. Cecilia Vogel Lecture 4

Review  Reflection  law of reflection  normal  Mirrors  images  images of images Outline  Refraction  Law of Refraction  Total internal reflection  Dispersion  prisms and rainbows  lens types

 We already know:  Light usually changes direction when entering new material  but ALWAYS goes to quadrant it’s headed for.  In what way and how much does direction change?  Given by Law of refraction  AKA Snell’s Law Law of Refraction n 1 sin  1 =n 2 sin  2

 If  1 =0, then  2 =0  light goes straight thru if perpendicular to surface  Otherwise…  If n 2 >n 1, then  1 >  2  BENDS TOWARD NORMAL  If n 1 >n 2, then  2 >  1  BENDS TOWARD SURFACE  Larger n  smaller angle Law of Refraction n 1 sin  1 =n 2 sin  2

 If  1 =0, then  2 =0  light goes straight thru perpendicular to surface  Otherwise…  Change in direction is greater for a greater change in n  Law of Refraction n 1 sin  1 =n 2 sin  2

Total Internal Reflection  No refracted ray!  Try sin  2 =n 1 sin  1 /n 2  but sin  2 can’t be greater than 1!  If RHS >1, there's no refraction, no  2  Fiber optics  Light reflects totally within fiber, does not refract out

 Critical angle (  c )  n 1 sin  c /n 2 = 1 is “critical”  just barely refracts   c = sin -1 (n 2 /n 1 ) ex:  c = sin -1 ( 1.0/1.5 ) = 42 o glass-air  Only TIR if n 1 >n 2 AND  1 >  c. Total Internal Reflection 42 o reflection refraction reflection only!

Dispersion  The index of refraction depends on wavelength  red vs violet :  violet has higher n in most materials  violet refracts more from air  Flat slab (e.g. window pane)  red and violet refract slightly differently  second surface un-does refraction  boring

Dispersion  Prism (or raindrop) -- surfaces not parallel  violet refracts more at first surface  even more at second surface  red and violet diverge – see a spectrum

Lenses  Two types  converging  center thicker  sometimes called convex or positive  diverging  edges thicker  sometimes called concave or negative

Converging Lens  Suppose light comes in perpendicular to lens.  What happens to it, if it passes through :  center  straight thru (  =0)  top of lens  bent downward  bottom  bent upward  Parallel Rays CONVERGE  Demo of converging lens f

Diverging Lens  Suppose light comes in perpendicular to lens.  What happens to it, if it passes through :  center  straight thru (  =0)  top of lens  bent upward  bottom  bent downward  Parallel Rays DIVERGE  Demo of diverging lens f (negative)