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# lec7 - Light propagation Ken Intriligators week 7 lectures...

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Light propagation Ken Intriligator’s week 7 lectures, Nov.12, 2013 Tuesday, November 12, 2013

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What is light? Old question: is it a wave or a particle? Quantum mechanics: it is both! 1600-1900: it is a wave. ~1905: photons Wave: wavelength determines color, amplitude determines intensity, brightness. Photons: massless particles, energy determined by the wavelength, color. Intensity determined by number of photons. Won’t discuss photons further in 2C. Tuesday, November 12, 2013
Physics 2B stuff: Changing B Feld acts as a source to make EM± (that’s how electricity is generated). Changing E Feld also acts as a source for B. Light is an E & B wave, each ²ip-²opping to make the other. Wave moves at v=c, the speed of light. ° k ° S ° E × ° B Tuesday, November 12, 2013

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(The details) Maxwell eqns: · ° E = ρ / ± 0 , ∇× ° E = ° B t · ° B =0 , ° B = μ 0 ° J + μ 0 ± 0 ° E t . Empty space, no sources 0 0 E.g. take ° E = ° E 0 cos( ° k · ° r ω t ) ° B = ° B 0 cos( ° k · ° r ω t ) · ° E = ( ° k · ° E 0 )sin( ° k · r ω t ) & ° E = ( ° k × ° E 0 ° k · r ω t ) Need: ° k · ° E 0 = ° k · ° B 0 E and B are transverse to the wave velocity. ° B 0 = ω 1 ° k × ° E 0 ω = c | ° k | c =1 / μ 0 ° 0 Tuesday, November 12, 2013
Example of light wave ° E = E 0 ° y cos( kx ω t ) “plane wave” moving in the + x direction. v φ ω /k = c Maxwell’s eqns. require Light wave carries energy and momentum (recall the Poynting vector). It can also carry angular momentum.

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lec7 - Light propagation Ken Intriligators week 7 lectures...

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