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LECTURE 23: Light Propagation of Light: Huygen’s Principle
plane
wave
spherical
wave
Each point on a primary wavefront serves as the
source of spherical secondary wavelets that
advance with a speed and frequency equal to those
of the primary wave. The primary wavefront at some
later time is the envelope of these wavelets.
Propagation of Light: Fermat’s Principle
The path taken by light traveling from one point
to another is such that the time of travel is a
minimum.
constant speed: minimize distance
traveled.
two different speeds: minimize distance
traveled at the slower speed.
4/9/12 4 Index of Refraction
4/9/12 5
Index of Refraction
medium index of refraction n vacuum exactly 1 air (STP) 1. H 2 O (20^0 C) 1. crown glass 1. diamond 2. Monochromatic: One wavelength Chromatic Dispersion: spreading of light according to its wavelength. 4/9/12 6
Geometric Optics
Material 1 Material 2 incident ray reflected ray refracted ray *Waves whose << than the objects with which it interacts. *Assume ligth propagates in a straight line (rays) 4/9/12 7 Reflection i r i -angle of incidence r - angle of reflection All measured relative to the normal to the interface. 4/9/12 8 Refraction 1 2 n 1 n 2 2
^1
n 1 n 2
4/9/12 13
Total Internal Reflection
4/9/12 14
Relative Intensity of Reflected & Refracted Light
For Normal Incidence: I = Io n 1 − n 2 n 1 + n 2 ⎛ ⎝⎜ ⎞ ⎠⎟ 2
