________________________________ From: James Propp <jpropp@cs.uml.edu> To: math-fun@mailman.xmission.com Sent: Friday, November 14, 2008 12:12:19 AM Subject: [math-fun] optics / materials science question While we're talking about optics, I was musing the other day about whether there's a way to make a windshield (or a pair of sunglasses) that will transmit less of the light emitted by really bright sources without transmitting less of the light emitted by dimmer sources. (I'm not talking about glare-reduction of the ordinary sort --- at least I don't think I am! --- since it's my impression that glare-reduction makes everything look dimmer.) It's not enough to make the windshield out of a substance that has non-constant transmittance (i.e., a substance for which the amount of transmitted light is some non-linear function of the amount of incident light), because every point on the windshield gets as many photons as every other; they're just travelling in different directions. We would need different transmittances in different directions. Are there materials that do this? Are such materials theoretically possible but not known? Or are there physical reasons why no such materials could exist? Jim Propp _______________________________________________ There are optical structures called photonic crystals. They have an internal structure whose optical properties vary periodically on a scale of the wavelength of light. Imagine, for example, a 3D checkerboard made of micrometer-sized cubes in which the red and black cubes are made of materials having two different refractive indices. Light transmission in a photonic crystal varies with wavelength, direction and polarization, possessing pass bands where the light is transmitted, ideally 100%, and stop bands where no light is transmitted, ideally 0%. It's plausible that someone could design a photonic crystal that would block light within a certain wavelength range and also within a certain solid angle of directions, and for any piolarization. The stop band directions are however fixed with respect to the photonic crystal, so this device will only block light in a fixed direction. It is not inconceivable that one might build a photonic crystal whose band structure could be modulated, electrically, optically, or acoustically. Then a camera and software could determine the direction of a bright source, and control the photonic crystal to block light from that direction. Gene