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11. Light Scattering

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11. Light Scattering 11. Light Scattering Coherent vs. incoherent scattering Radiation from an accelerated charge Larmor formula Why the sky is blue Rayleigh scattering Reflected and refracted beams from water droplets Rainbows Coherent vs. Incoherent light scattering Coherent light scattering: scattered wavelets have nonrandom relative phases in the direction of interest. Incoherent light scattering: scattered wavelets have random relative phases in the direction of interest. Example: Randomly spaced scatterers in a plane Incident Incident wave wave Forward scattering is coherent— Off-axis scattering is incoherent even if the scatterers are randomly when the scatterers are randomly arranged in the plane. arranged in the plane. Path lengths are equal. Path lengths are random. Coherent vs. Incoherent Scattering N Incoherent scattering: Total complex amplitude, Aincoh exp(j m ) (paying attention only to the phase m1 of the scattered wavelets) The irradiance: NNN2 2 IAincoh incohexp( j m ) exp( j m ) exp( j n ) mmn111 NN NN exp[jjN (mn )] exp[ ( mn )] mn11mn mn 11 mn m = n mn Coherent scattering: N 2 2 Total complex amplitude, A coh 1 N . Irradiance, I A . So: Icoh N m1 So incoherent scattering is weaker than coherent scattering, but not zero. Incoherent scattering: Reflection from a rough surface A rough surface scatters light into all directions with lots of different phases. As a result, what we see is light reflected from many different directions. We’ll see no glare, and also no reflections. Most of what you see around you is light that has been incoherently scattered. Coherent scattering: Reflection from a smooth surface A smooth surface scatters light all into the same direction, thereby preserving the phase of the incident wave. How smooth does the surface need to be? To be smooth, the roughness needs to be smaller than the wavelength of the light. As a result, images are formed by the reflected light. Wavelength-dependent incoherent scattering: Why the sky is blue Air molecules scatter light, and the scattering depends on frequency. Light from the sun Air Shorter-wavelength light is scattered out of the beam, leaving longer- wavelength light behind, so the sun appears yellow. In space, the sun is white, and the sky is black. Radiation from an accelerated charge In order to understand this scattering process, we will analyze it at a microscopic level. With several simplifying assumptions: 1. the scatterer is much smaller than the wavelength of the incident light 2. the frequency of the light is much less than any resonant frequency. ct coasting at constant velocity v for a time t1 tiny period of { acceleration, { r = ct of duration t 1 initial position of a charge q, at rest Radiation from an accelerated charge E vt By similar triangles: 1 E|| ct E E|| But the velocity v can be related to the acceleration during the small interval t: vt 1 ct v = a t vt1 vt|| 1 which implies: vat at ar and therefore: 1 EE || E || 2 cc Finally, the field E || must be equal to the field of a static charge (this can be proved using Gauss’ Law): q qa E|| E 4r 2 2 0 4rc 0 Radiation from an accelerated charge q qa E|| E 4r 2 2 0 4rc 0 10 0 As r becomes large, the parallel 10 -1 component goes to zero much 10 -2 1/r more rapidly than the perpendicular -3 component. We can therefore 10 10 -4 2 neglect E|| if we are far enough 1/r away from the moving charge. 10 -5 -6 10 0 1 2 3 4 5 6 7 8 9 10 Also: aasin So, the radiated EM wave has a magnitude: qtasin Ert, 2 4rc 0 Spatial pattern of the radiation qt22asin 2 Magnitude of the Poynting vector: 2 Srt,sin223 160 r c direction of the acceleration a 0 330 30 300 60 270 S 90 240 120 210 150 180 2D slice 3D cutaway view No energy is radiated in the direction of the acceleration. Total radiated power - the Larmor formula To find the total power radiated in all directions, integrate the magnitude of the Poynting vector over all angles: 2 Pt r2 sin d d Srt , 00 q22a sin3 d 3 8c 0 0 This integral is equal to 4/3 Sir Joseph Larmor 1857-1942 q22a Thus: Pt 3 6c 0 . This is known as the Larmor formula (1897) . Total radiated power is independent of distance from the charge . Total power proportional to square of acceleration Larmor formula - application to scattering Recall our derivation of the position of an electron, bound to an atom, in an applied oscillating electric field: eE m x te 0 e jt (we can neglect the damping e 2 2 factor , for this analysis) 0 We assume that the light wave frequency is much smaller than the resonant frequency, << 0, so this is approximately: eE0 me jt xe te 2 0 From the position we can compute the acceleration: 2 2 dxe jt atee22 eEm0 e This is known as dt 0 Rayleigh scattering: Insert this into the Larmor formula to find: scattered power proportional to 4 24 (Rayleigh: 1871) Pascat e P incident This is (mostly) why the sky is blue. th Rayleigh Scattering: Total scattered power ~ 4 power of the frequency of the incident light Blue light ( = 400 nm) is scattered 16 times more efficiently than red light ( = 800 nm) scattered light that we see sunlight earth For the same reason, People here see the sunsets are red. unscattered remaining light The world of light scattering is a very large one Particle size/wavelength ~0 ~1 Large There are many Rayleigh-Gans Scattering regimes of particle scattering, depending on the particle size, the ~0 light wavelength, Mie Scattering and the refractive index. Refractive index As a result, there Geometrical optics Rayleigh Scattering are countless observable effects Totally reflecting objects of light scattering. Large ~1 Another example of incoherent scattering: rainbows water droplet Input light paths Light can Path leading enter a to minimum deflection droplet at different ~180° deflection distances from its edge. One can compute the deflection angle of the emerging light Minimum deflection as a function of the angle (~138°) deflection angle incident position. (relative to the original direction) Deflection angle vs. wavelength Because n varies with wavelength, the minimum deflection angle varies with color. Lots of violet deflected at this angle Lots of red deflected at this angle Lots of light of all colors is deflected by more than 138°, so the region below rainbow is bright and white. The size of rainbows If the light source is lower than the viewer’s perspective, then you can see more than half an arc. The minimum deviation angle of 138 is what determines the size of the circle seen by the viewer: 180 – 138 = 42 opening angle. A rainbow, with supernumeraries The sky is much brighter below the rainbow than above. The multiple greenish-purple arcs inside the primary bow are called “supernumeraries”. They result from the fact that the raindrops are not all the same size. In this picture, the size distribution is about 8% (std. dev.) Explanation of 2nd rainbow A 2nd rainbow can result from light entering the droplet in its lower half and making 2 internal reflections. Water droplet Deflection angle Minimum deflection angle (~232.5°) yielding a rainbow radius of 52.5°. Distance from droplet edge Because the angular radius is larger, the 2nd bow is above the 1st one. Because energy is lost at each reflection, the 2nd rainbow is weaker. Because of the double bounce, the 2nd rainbow is inverted. And the region above it (instead of below) is brighter. “ray tracing” A double rainbow Note that the upper bow is inverted. The dark band between the two bows is known as Alexander’s dark band, after Alexander of Aphrodisias who first described it (200 A.D.) Multiple order bows 3 5 4 6 A simulation of the Ray paths for the higher order bows higher order bows • 3rd and 4th rainbows are weaker, more spread out, and toward the sun. • 5th rainbow overlaps 2nd, and 6th is below the 1st. • There were no reliable reports of sightings of anything higher than a second order natural rainbow, until… The first ever photo of a triple and a quad (involving multiple superimposed exposures and significant image processing) from “Photographic observation of a natural fourth-order rainbow,” by M. Theusner, Applied Optics (2011) Other atmospheric optical effects Look here for lots of information and pictures: http://www.atoptics.co.uk Six rainbows? Explanation: http://www.atoptics.co.uk/rainbows/bowim6.htm.
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