11Where does the word "laser" come from?A: The name of the physicist who invented it.B: The name of the dog of the physicist who invented itC: "Light Amplification by Stimulated Emission of Radiation"D: It's a mix of "light" and "phaser"E: It was just made up because it sounded cool2CAPA #14 Tonight. Set #15 in bins - short!No Tut HW for Tues (yay!) BUT there IS a pretest, as usualReading: (If you haven’t already -> ) 33.9, 34.7, 8There is a new (and final!) online participation available -don’t forget about it!_________________________Last: Maxwell's equations, light, and EM radiationToday: More of Maxwell and EM radiationNext: Waves, LC circuits and resonance. (Prof. Gurarie)3Maxwell’s equations.The axioms ofelectromagnetism.They are one of themajor triumphs ofmodern science! ! r E • dv A ""= Qenclosed/#0r B • dv A ""= 0r E • dr l = $d%Bdt"r B • dr l =µ0ithrough+µ0#0d%Edt"24Visible Light:Example: Yellow lightFrequency = 5E14 Hzλ = 500-550 nm (yellow)5Traveling wavex(position)λf λ = cλ /T = cWavelength6Visible Light: λ= 400 nm (violet) 700 nm (red)Example:λ = 550 nm (yellow)f = c/λ = 3x108/550x10-9 = 5x1014 Hzf λ = c37Television transmissions are in the“radio” range.λ = 1-5 metersf =c/λ ~ 108 Hz = 100 MHzOn Television, different channels havedifferent frequencies and thus differentwavelengths. When you set thechannel knob, you amplify just thatfrequency range.8A radio wave of wavelength 2 meters passes by a person witha radio receiver. The person “watches” the electric andmagnetic fields go up and down as the wave travels past.After 1 second, the number of waves that moved past theperson is:A: 1 waveB: 3x108 wavesC: 1.5x108 wavesD: 6x108 wavesE: Not enough information9A radio wave of wavelength 2 meters passes by a person witha radio receiver. The person “watches” the electric andmagnetic fields go up and down as the wave travels past.After 1 second, the number of waves that moved past theperson is:A: 1 waveB: 3x108 wavesC: 1.5x108 wavesD: 6x108 wavesE: Not enough information3E8 m/s = lambda*f = 2m *fSo f = 1.5E8 HzThat’s 1.5E8 cycles each second410A radio wave of wavelength 2 meters passes by a person witha radio receiver. The person watches the E and B fields go upand down as the wave travels by. Later, a new radio wavepasses the person. They observe that the E and B fields go upand down 10 times faster than the original wave.What is the best conclusion? The second wave…A: has a wavelength of 0.2 mB: has a wavelength of 20 mC: travels 10 times faster.D: has 1/10 the frequency.E: More than one (or none of these) is correct.11Polarization of LightEM waves have a direction of the Electric field vector.EBcAs viewed from an observer:EB12Ordinary Light (from the Sun or a lightbulb) is unpolarized,meaning it is a mixture of waves with Electric field vectors inrandom directions within the plane perpendicular to the wavedirection. c c UnpolarizedPolarized513Polarizer = Polaroid filter = filter that passes light with theE-field along the “pass axis” of the filter only. c PolarizedE FilterLight Passes Through…14 c PolarizedE FilterLight Does Not Pass Through…All the EM energy is used up aswork in moving chargedparticles in the filter.Polarizer = Polaroid filter = filter that passes light with theE-field along the “pass axis” of the filter only.15If the E-field is not all parallel to the pass axis, only thecomponent of E along the pass axis gets through.EθFilter AxisOnly Ecosθ is transmitted.intensity is related to ENERGYAnd thus to field strength squared:)(cos20!SStrans=616An unpolarized beam of light passes through a Polaroidfilter. The intensity of the original beam is Io. What is theintensity of the light coming through the filter?A: (1/1.4)Io B: (1/2)Io C: (1/4)Io D: Io E: None of theseAnswer: (1/2)Io After the first filter, the intensity is I1 = Io(1/2). Reason:If the original beam were polarized, then after passing through one filterthe intensity would be I1 = Io cos2θ , but here the original beam isunpolarized so all angles θ are in the beam. The intensity after one filteris then I1 = Io (cos2θ)average = Io(1/2).Io c Clicker Question17An unpolarized beam of light passes through 2 Polaroidfilters oriented at 45o with respect to each other. Theintensity of the original beam is Io. What is the intensity ofthe light coming through both filters?A: (1/1.4)Io B: (1/2)Io C: (1/4)Io D: (1/8)Io E: None Answer: (1/4)Io After the first filter, the intensity is I1 = Io(1/2).After the second filter the intensity is I2 = I1 cos245 = I1 (1/2) =Io(1/2)(1/2) = Io(1/4).Io c Clicker Question18An unpolarized beam of light passes through 3 Polaroid filters,each oriented at 45o with respect to each other. The intensity ofthe original beam is Io.Does ANY light pass through the last filter?A: Yes, some makes it thruB: None makes it thruIo c719A. a = d > b = c B. b = c > a= d C. d > a > b >c D. d > a > b = cE. Something else Unpolarized light of equal intensityreaches four pairs of polarizing filters.Rank in order, (largest to smallest) theintensities transmitted through thesecond polarizer of each pair.abcd20Why do polarizing filter sunglasses work?If sun light is unpolarized, then it justmakes things darker by removing onecomponent.However, scattering can polarize light.Then the sunglasses work very wellat reducing this light (glare).23Who are these people and what do they have in
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