Electromagnetic WavesPAL #20 EM RadiationSlide 3Slide 4Slide 5PolarizationPolaroidPolarization and IntensityIncident Polarized LightMultiple SheetsSheet AnglesMeans of PolarizationSlide 13Reflection and RefractionGeometryLawsIndex of RefractionGeneral CasesTotal Internal ReflectionChromatic DispersionSlide 21Polarization By ReflectionBrewster AngleNext TimeElectromagnetic Waves Physics 202Professor Lee CarknerLecture 21PAL #20 EM RadiationAcceleration of lightsail craftF = ma = prA a = prA/mpr = 2I/cI = Ps/4r2 = (3.9X1026)/( 4 (1.5X1011)2) = 1379 Wpr = (2)(1379)/(3X108) = 9.2X10-6 N/m2a = (9.2X10-6)(2.25X108)/5000 = 0.41 m/s2Time to get to moond = ½at2t = (2d/a)½ = [(2)(3.8X108)/(0.41)] ½t = 43054 sec ~ 12 hoursProblemsSunlight only pushes in one directionHow do you stop or go back?Gravity and inherited motion also importantConsider a dust grain near a star. If the grain is perfectly balanced between light pressure out and gravity in, what happens to the grain if the mass doubles (but the size stays the same)?A) Goes inB) Goes outC) Stays putConsider a dust grain near a star. If the grain is perfectly balanced between light pressure out and gravity in, what happens to the grain if the mass doubles and the surface area doubles?A) Goes inB) Goes outC) Stays putConsider a dust grain near a star. If the grain is perfectly balanced between light pressure out and gravity in, what happens to the grain if the distance from the star doubles?A) Goes inB) Goes outC) Stays putPolarization The plane containing the E vectors is called the plane of oscillation Most light sources are unpolarizedAny given wave has a random plane of oscillationPolaroid Polaroid is a sheet of material that will only pass through the components of the E vectors in a certain direction If you put a horizontal Polaroid sheet on top of a vertical Polaroid sheet no light gets throughPolarization and Intensity The sum of all of the y components should be equal to the sum of all of the z components I = ½ I0This is true only when the incident light is completed unpolarizedWhat about polarized light hitting Polaroid?Incident Polarized LightFor polarized light incident on a sheet of Polaroid, the resultant intensity depends on the angle between the original direction of polarization and the sheet E = E0 cos I = I0 cos2 For unpolarized light that pass through two polarizing sheets, is the angle between the two sheetsMultiple SheetsSheet AnglesMeans of Polarization This alignment permits only the components in that direction to pass The dust grains are partially aligned by the galactic magnetic field and so the light is partially polarized Light can also be polarized by reflectionReflection and RefractionWhen light passes from one medium to another (e.g. from air to water) it will generally experience both reflection and refraction Refraction is the bending of the portion of the light that does penetrate the surfaceGeometry AnglesAngle of incidence (1): the angle between the incident ray and the normal Angle of reflection (1’): Angle of refraction (2): the angle of the refracted ray and the normalLawsLaw of Reflection Law of Refraction n2 sin 2 = n1 sin 1Where n1 and n2 are the indices of refraction of the mediums involvedIndex of RefractionEvery material has an index of refraction that determines its optical properties n is always greater than or equal to 1Large n means more bendingGeneral Casesn2 = n1 2 = 1e.g. air to airn2 > n1 2 < 1e.g. air to glassn2 < n1 2 > 1e.g. glass to airTotal Internal ReflectionConsider the case where 2 = 90 degrees For angles greater than 90 there is no refraction and the light is completely reflectedn1 sin c = n2 sin 90c = sin-1 (n2/n1)This is the case of total internal reflection, where no light escapes the first mediumChromatic Dispersion In general, n is larger for shorter wavelengths Incident white light is spread out into its constituent colorsChromatic dispersion with raindrops causes rainbowsChromatic DispersionPolarization By ReflectionLight reflected off of a surface is generally polarized When unpolarized light hits a horizontal surface the reflected light is partially polarized in the horizontal direction and the refracted light is partially polarized in the vertical directionBrewster Angle At B the reflected and refracted rays are perpendicular to each other, soB + r = 90 B = tan-1 (n2/n1)If we start out in air n1 = 1 so:B = tan-1 nNext TimeRead: 34.1-34.6Homework: Ch 33, P: 37, Ch 34, P: 7, 9, 10, 11, 12, 13,
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