Geometrical Optics LL2 Section 53

Local propagation vector is perpendicular to wave surface Looks like a plane wave if amplitude and direction are ~constant over distances of at least a wavelength Arbitrary electromagnetic wave Curve whose tangent at each point corresponds to propagation direction Wave surfaces: phase of the wave is the same on each such surface

Geometrical optics holds in limit 0, when propagation of rays is divorced from wave properties Let f be any field component of the wave. For plane monochromatic wave constant

In general case, let f = a e i  The phase  is called the “eikonal” If the wave is not a plane wave, then a = a(r,t), i.e. not constant, and   -k i x i + .  changes by 2  when we move through one wavelength. Since  0 for geometrical optics,  must be large.

Choose an origin in 4-space. Near the origin, r and t are small. Expand eikonal in powers of coordinates As in any small region, wave looks like a plane wave, so Eikonal equation: Fundamental equation of geometrical optics

The 4D wave equation is Substitute f = a e i  The last term is 2 nd order in the large quantity . All other terms are relatively small. Again we get the eikonal equation.

The Hamilton-Jacobi equation for a particle in given EM-fields involves first partial derivatives of the action S and is second order in S For a free particleand Compare these to and (16.11)

Propagation of particlesPropagation of rays

In vacuum The wavevector is constant Rays are straight lines, and propagation speed is c

A wavepacket is a superposition of monchromatic waves with a small range of frequencies that occupies a finite region of space (48.15) For each monochromatic component Replace k i by average values for the wavepacket Compare 4-momentum for massless particle Wavepacket average energy and momentum transform like frequency and wavevector

A Lagrangian for rays doesn’t exist

Time dependence for a wave with definite constant frequency is given by e -i  t If Then eikonal must be = constant

If  is definite and constant, then wave surfaces are surfaces of constant  0 Space part of f = a e i   must be constant Then

When energy is constant, Hamilton’s principle becomes Maupertius’s principle in mechanics Integral is over the trajectory of a particle between two points, where

In optics, Maupertius’s principle becomes Fermat’s principle Minimum path between two points is a straight line. Fermat’s principle gives rectilinear propagation of rays in vacuum Ray seeks the minimum path

Assignments (10 min white board presentations) Prepare 10 minute white-board presentation on the transition from quantum mechanics to classical mechanics, see LL3, secs 6 and 17. Prepare 10 minute white-board presentation on Maupertius’s principle, see LL1, sec 44.