Physics 2102 QuickTime and a decompressor are needed to see this picture Jonathan Dowling Lecture 24 TUE 20 APR 2010 Ch 33 6 10 E M Waves QuickTime and a decompressor are needed to see this picture QuickTime and a decompressor are needed to see this picture Radiation Pressure Waves not only carry energy but also momentum The effect is very small we don t ordinarily feel pressure from light If light is completely absorbed during an interval Dt the momentum transferred is given by u p and twice as much if reflected Newton s law p F t c Now supposing one has a wave that hits a surface of area A perpendicularly the amount of energy transferred to that surface in time Dt will be U IA t Radiation pressure IA t p therefore c F A I IA F c I 2I pr total absorption pr total reflection Pa N m2 c c Radiation Pressure Comet Tails QuickTime and a decompressor are needed to see this picture Solar Sails Photons Propel Spacecraft QuickTime and a decompressor are needed to see this picture StarTrek DS9 QuickTime and a decompressor are needed to see this picture NASA Demo QuickTime and a decompressor are needed to see this picture NASA Concept QuickTime and a decompressor are needed to see this picture EM waves polarization Radio transmitter If the dipole antenna is vertical so will be the electric fields The magnetic field will be horizontal he radio wave generated is said to be polarized In general light sources produce unpolarized waves emitted by atomic motions in random directions EM Waves Polarization Completely unpolarized light will have equal components in horizontal and vertical directions Therefore running the light through a polarizer will cut the intensity in half I I0 2 When polarized light hits a polarizing sheet only the component of the field aligned with the sheet will get through E y E cos And therefore I I 0 cos 2 Example Initially unpolarized light of intensity I0 is sent into a system of three polarizers as shown What fraction of the initial intensity emerges from the system What is the polarization of the exiting light Through the first polarizer unpolarized to polarized so I 1 I0 Into the second polarizer the light is now vertically polarized Then I2 I1cos260o 1 4 I1 1 8 I0 Now the light is again polarized but at 60o The last polarizer is horizontal so I3 I2cos230o 3 4 I2 3 32 I0 0 094 I0 The exiting light is horizontally polarized and has 9 of the Reflection and Refraction When light finds a surface separating two media air and water for example a beam gets reflected bounces and another gets refracted bends Law of reflection Light Bounces the angle of incidence 1 equals the angle of reflection 1 1 1 Law of Refraction Light Bends n2 sin 2 n1 sin 1 Snell s Law n is the index of refraction of the medium n vacuum n 1 In air n 1 In all other media n 1 Plastic Speed of Light is Slowed n 1 Glass n 1 n 1 QuickTime and a decompressor are needed to see this picture Hits Sand Turns Left Road v c n n 1 v c vL v Example Water has n 1 33 How much does a beam incident at 45o refracts n2 sin 2 n1 sin 1 sin 2 n1 n2 sin 1 1 1 33 sin 45o 0 0098 2 32o QuickTime and a decompressor are needed to see this picture QuickTime and a decompressor are needed to see this picture Water on Desert Road Illusion The index of refraction decreases with temperature the light gets refracted and ends up bending upwards We seem to see water on the road but in fact we are looking at the sky Water on the Desert Road Mirage QuickTime and a decompressor are needed to see this picture Chromatic Dispersion The index of refraction depends on the wavelength color of the light QuickTime and a decompressor are needed to see this picture The Single Rainbow QuickTime and a decompressor are needed to see this picture The Double Rainbow Total Internal Reflection QuickTime and a decompressor are needed to see this picture Total Internal Reflection From glass to air the law of refraction uses n2 n1 so 2 1 it may reach 90o or more the ray is reflected instead of refracted 2 Condition for TIR n2 sin 2 n1 sin 1 2 90 n2 1 1 n1 1 For glass fused quartz n 1 46 and the critical angle is 43o optical fibers Fish Underwater Can t See Entire Sky QuickTime and a decompressor are needed to see this picture Total Internal Reflection Zone QuickTime and a decompressor are needed to see this picture The cut of the diamond favors total internal reflection Most rays entering the top of the diamond will internally reflect until they reach the top face of the diamond where they exit This gives diamonds their bright sparkle A fiber optic is a glass hair which is so thin that once light enters one end it can never strike the inside walls at less than the critical angle The light undergoes total internal reflection each time it strikes the wall Fiber optic cables are used to carry telephone and computer communications Fiber Optic Cables QuickTime and a decompressor are needed to see this picture QuickTime and a decompressor are needed to see this picture Underwater Telephone and Internet Fiber Telecom Cables QuickTime and a decompressor are needed to see this picture FIBER OPTIC GUIDED MISSILE QuickTime and a decompressor are needed to see this picture QuickTime and a decompressor are needed to see this picture Polarization By Reflection Different polarization of light get reflected and refracted with different amplitudes birefringence At one particular angle the parallel polarization is NOT reflected at all This is the Brewster angle B and B r 90o o n1 sin n2 sin 90 n2 cos n2 tan n1 Polarizing Sunglasses Polarized Sunglasses QuickTime and a decompressor are needed to see this picture QuickTime and a decompressor are needed to see this picture
View Full Document
Unlocking...