Where 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 it C 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 cool 1 CAPA 14 Tonight Set 15 in bins short No Tut HW for Tues yay BUT there IS a pretest as usual Reading If you haven t already 33 9 34 7 8 There is a new and final online participation available don t forget about it Last Maxwell s equations light and EM radiation Today More of Maxwell and EM radiation Next Waves LC circuits and resonance Prof Gurarie 2 r v E dA Q r v B dA 0 enclosed r r r r E dl B dl Maxwell s equations 0 The axioms of electromagnetism d B dt 0ithrough 0 0 d E dt They are one of the major triumphs of modern science 3 1 Visible Light Example Yellow light Frequency 5E14 Hz 500 550 nm yellow 4 Traveling wave Wavelength x position T c f c 5 Visible Light 400 nm violet 700 nm red Example f c 550 nm yellow f c 3x108 550x10 9 5x1014 Hz 6 2 Television transmissions are in the radio range 1 5 meters f c 108 Hz 100 MHz On Television different channels have different frequencies and thus different wavelengths When you set the channel knob you amplify just that frequency range 7 A radio wave of wavelength 2 meters passes by a person with a radio receiver The person watches the electric and magnetic fields go up and down as the wave travels past After 1 second the number of waves that moved past the person is A 1 wave B 3x108 waves C 1 5x108 waves D 6x108 waves E Not enough information 8 A radio wave of wavelength 2 meters passes by a person with a radio receiver The person watches the electric and magnetic fields go up and down as the wave travels past After 1 second the number of waves that moved past the person is 3E8 m s lambda f A 1 wave 2m f B 3x108 waves So f 1 5E8 Hz C 1 5x108 waves That s 1 5E8 cycles each second D 6x108 waves E Not enough information 9 3 A radio wave of wavelength 2 meters passes by a person with a radio receiver The person watches the E and B fields go up and down as the wave travels by Later a new radio wave passes the person They observe that the E and B fields go up and down 10 times faster than the original wave What is the best conclusion The second wave A has a wavelength of 0 2 m B has a wavelength of 20 m C travels 10 times faster D has 1 10 the frequency E More than one or none of these is correct 10 Polarization of Light EM waves have a direction of the Electric field vector As viewed from an observer E E B c B 11 Ordinary Light from the Sun or a lightbulb is unpolarized meaning it is a mixture of waves with Electric field vectors in random directions within the plane perpendicular to the wave direction Unpolarized c Polarized c 12 4 Polarizer Polaroid filter filter that passes light with the E field along the pass axis of the filter only E Filter Polarized Light Passes Through c 13 Polarizer Polaroid filter filter that passes light with the E field along the pass axis of the filter only E Filter Polarized Light Does Not Pass Through c All the EM energy is used up as work in moving charged particles in the filter 14 If the E field is not all parallel to the pass axis only the component of E along the pass axis gets through Only Ecos is transmitted E intensity is related to ENERGY And thus to field strength squared Filter Axis Strans S 0 cos 2 15 5 Clicker Question An unpolarized beam of light passes through a Polaroid filter The intensity of the original beam is Io What is the intensity of the light coming through the filter Io A 1 1 4 Io B 1 2 Io C 1 4 Io D Io E None of these c Answer 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 filter the intensity would be I1 Io cos2 but here the original beam is unpolarized so all angles are in the beam The intensity after one filter is then I1 Io cos2 average Io 1 2 16 Clicker Question An unpolarized beam of light passes through 2 Polaroid filters oriented at 45o with respect to each other The intensity of the original beam is Io What is the intensity of Io the light coming through both filters A 1 1 4 Io B 1 2 Io C 1 4 Io D 1 8 Io E None c 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 17 An unpolarized beam of light passes through 3 Polaroid filters each oriented at 45o with respect to each other The intensity of the original beam is Io Does ANY light pass through the last filter Io c A Yes some makes it thru B None makes it thru 18 6 a b Unpolarized light of equal intensity reaches four pairs of polarizing filters Rank in order largest to smallest the intensities transmitted through the second polarizer of each pair d c A a d b c B b c a d C d a b c D d a b c E Something else 19 Why do polarizing filter sunglasses work If sun light is unpolarized then it just makes things darker by removing one component However scattering can polarize light Then the sunglasses work very well at reducing this light glare 20 Who are these people and what do they have in common 23 7
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