Phy107Spr06Lect15

Phy107Spr06Lect15 - From Last Time E le c t ric it y a n d...

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1 Wed. Feb. 22, 2006 Phy107 Lecture 15 1 From Last Time… • Electricity and magnetism intimately related. • Electric charges can be either positive or negative. • Magnetic ‘charges’ are dipoles, combined north (+) and south (-) poles. • An electric current generates a magnetic field. • A time-varying magnetic field generates an electric field. • Both electric and magnetic fields are present in an electromagnetic wave. Today: properties of EM waves • Interference of EM waves • Color and perception of light waves Wed. Feb. 22, 2006 Phy107 Lecture 15 2 Wave interference • Interference arises when waves change their ‘phase relationship’. • Can vary phase relationship of two waves by changing physical location of speaker. Constructive Destructive in-phase’ ‘1/2 λ phase diff’ Wed. Feb. 22, 2006 Phy107 Lecture 15 3 Interference of light waves Can be constructive or destructive Depends on whether crests or troughs coincide. As in sound waves, crests and troughs become separated due to different propagation distances. But wavelength is very small (visible light wavelength ~ 1/50 human hair diameter). Effects more subtle than in sound waves. But this also means that interference can be used to measure very small changes in distance. Wed. Feb. 22, 2006 Phy107 Lecture 15 4 Constructive and destructive Equal path lengths Constructive interference Path lengths different by 1/2 wavelength Destructive interference Wed. Feb. 22, 2006 Phy107 Lecture 15 5 Interference of 2 phase coherent sources constructive interference destructive interference Wed. Feb. 22, 2006 Phy107 Lecture 15 6 Resulting diffraction pattern
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2 Wed. Feb. 22, 2006 Phy107 Lecture 15 7 Question A red laser pointer is shone on two slits, and a diffraction pattern is observed on the wall. At a bright spot, the distances to the two slits are 1.0000006 meters and 1.0000000 meters. What could be the wavelength of the laser pointer? A. 600 nm B. 6000 nm C. 3000 nm The difference in the two path distances from the bright spot to the two slits is 0.0000006 m = 6x10 -7 m = 600 nm. For constructive interference, this path difference must be an integer number of wavelengths (e.g. 1, 2, 3, etc.). If this is 1 wavelength, then the wavelength is 600 nm. If 2 wavelengths, wavelength = 300nm, etc. Wed. Feb. 22, 2006 Phy107 Lecture 15 8 Laser pointer interference Each clear area on the slide acts as a light source. Interference with many light sources is sometimes called diffraction. Wed. Feb. 22, 2006 Phy107 Lecture 15 9 Complex interference patterns White spaces act as array of sources. The ‘diffraction pattern’ contains information about the original pattern. Wed. Feb. 22, 2006
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This note was uploaded on 01/05/2012 for the course PHYS 107 taught by Professor N/a during the Spring '08 term at Wisconsin.

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Phy107Spr06Lect15 - From Last Time E le c t ric it y a n d...

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