PHYS 2020 Spring 2012 Announcements

Transcription

1 PHYS 2020 Spring 2012 Announements Continuing to adjust the shedule to relet the progress o the letures: HW #7 is now due Mon. Apr 9 1

2 Chapter 24 Eletromagneti Waes Next 3 hapters on the behaior o light 2

3 24.1 The Nature o Eletromagneti Waes y x Two straight wires onneted to the terminals o an AC generator an reate an eletromagneti wae. Only the eletri wae traeling to the right (x diretion) is shown here: we hae an eletri ield that alternates up and down (in the y diretion) at a gien point. And the loation o the peak ield moes to the right at speed = m/s. The urrent used to generate the eletri wae reates a magneti wae -- o alternating magneti ield (in y the z-diretion perpendiular to both the diretion o propagation and to the eletri x wae) that traels at the same speed. z 3

4 24.1 The Nature o Eletromagneti Waes This piture shows the wae o the radiation ield ar rom the antenna. z y x Eletri ield 2x E( x, t) E t ˆ 0 sin 2 je0 2x B( x, t) B 0 sin 2t kˆ B0 E0 B0 E B, 2x sin 2t 2x sin 2t, are mutually perpendiular and related by RHR-1 E Magneti ield ˆi B 4

5 24.1 The Nature o Eletromagneti Waes In Class Demo o Eletri Wae antenna powering light-bulbs at 60 Hz. A radio wae an be deteted with a reeiing antenna wire that is parallel to the eletri ield. With a reeiing antenna in the orm o a loop, the magneti ield o a radio wae an be deteted (loop oriented perpendiular to the magneti iled). 5

6 24.2 The Eletromagneti Spetrum Generated by osillating eletri/eletroni iruits Generated in nulear deay proesses and high-energy partile ollisions Like all waes, eletromagneti waes hae a waelength and requeny, related by the uniersal wae equation: m/s 6

7 24.2 The Eletromagneti Spetrum Visible, Inra-Red (IR), and ultra-iolet (UV) light all represent examples eletromagneti waes. They are generated in atomi (not nulear) and moleular transitions, reations and ollision proesses et. Example : Waelength Range o Visible Light Find the range in waelengths or isible light in the requeny range between 4.0x10 14 Hz and 7.9x10 14 Hz m/s m s Hz m 750 nm m s Hz m 30 nm Visible light: nm: This is why nobody makes mirosope objetie lenses with magniiation > 100x. One physially annot separate two dots that are within ~ o light used. With eye piee o 10x 500 nm at 1000x gies 0.5 mm. 7

8 24.3 The Speed o Light Measuring the Speed o Light Method o A. Mihelson (between ) Setup between Mt. Wilson and Mt. San Antonio near Los Angeles, CA based on earlier ideas o Fizeau and Fouault. Commemoratie display is loated at the Cii Center in Irine, CA) km Set up -sided mirror symmetrially to see light ater 3 reletions. The seond reletion is rom a stationary mirror 35.5 km away (on the other mountain) Start turning the otagon at ixed. The obserer no longer sees light unless the otagon turns a whole multiple o 1/ reolutions in the time t or light to trael the 71.0 km round-trip. Explanation: Light rom 1 st reletion will hit the ar mirror ONLY i the otagon is in the symmetri position short pulses releted rom ar mirror. And Ater the 2 nd reletion, the obserer will only see the releted light (ater 3 rd reletion) only i the otagon is one again in the symmetri position (i.e. whole multiples o 1/ reolutions). Illustratie Animations at: Result: minimum required = 52 re/s t T s -1 (52 s ) 3 4 ( m) /( s) m/s

9 24.3 The Speed o Light The speed o light in a auum m s The meter was redeined in 193 to make this exat In 161, James Clerk Maxwell (then a high-shool teaher in Sotland) published a paper in whih he generalized the eletromagneti ormulations in 4 equations. Cal 3: Deriaties in 3-D Gauss Law Coulomb s Law No magneti monopoles (no solated N or S poles) Faraday s Law o eletromagneti indution Ampère s Law Combining these 4 equations gies the lassial wae equations 2 1/ E 0 0 E and B 0 0 B 2 t t Maxwell s predition o the speed o eletromagneti waes o o C N m 4 10 Tm A m s 9

10 Light, and EM waes in general is a transerse wae The orientation o the eletri etor is known as the polarization We hae shown sop ar waes linearly poiarized in the y-diretion There are ilters, known as polarizers, whih an selet light o a partiular polarization. These are sheets o a material made o small linear antennae all oriented in the same diretion. The eletrons in the antenna is subjet to the ore o the omponent o the osillating eletri ield along the diretion o the antenna. The osillation o the eletrons then re-transmits the eletri wae along its own diretion and in doing so, polarizes the light along its own diretion. 10

11 24.5 The Doppler Eet and Eletromagneti Waes Eletromagneti (EM) waes also an exhibit a Doppler eet: 1. Inrease in obsered requeny or soure and obserer approahing one another 2. Derease in obsered requeny or soure and obserer reeding rom one another But Doppler Shit o EM waes diers rom that o sound or two reasons: a) Sound waes traels in a medium, whereas eletromagneti waes an trael in a auum (and in some transparent medium) b) For sound, it is the motion relatie to the medium that is important. For eletromagneti waes, only the relatie motion o the soure and obserer is important. o s 1 1 For sound Not or light o s o Obsered requeny Eletromagneti waes s 1 Soure requeny rel i rel Very muh less than 11