LECT19_21B

LECT19_21B - Maxwells Equations Predict Speed of EM Waves...

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Maxwell’s Equations Predict Speed of EM Waves / B = c
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Measuring c O. Roemer (1670s, Denmark): Times of observed eclipses of Jupiter’s moons deviated from periodic; deviation varied with Earth–Jupiter distance. c ~ 2.3 x 10 8 m/s richweb .f9.co.uk ~16 lt.- mins
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Measuring c 1728: J. Bradley: stellar aberration (changes in apparent position of stars) due to Earth’s rotation: c ~ 295,000 km/s Rotating mirrors: Foucault (1862), Michelson (1879): c ~ 300,000 km/s 1846: H.L. Fizeau: shining light between the teeth of a rapidly-rotating toothed wheel: c ~313,000 km/s Light Source stationary mirror rotating mirror
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The Electromagnetic Spectrum ALL of these are forms of EM radiation, differing only in λ or f: c = f λ Visible light: Blue/Violet light: λ ~ 400 nm, f ~ 7x10 14 Hz Red light: λ ~ 700nm, f ~ 4x10 14 Hz
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The Electromagnetic Spectrum Microwaves: cell phones, communication: λ ~ few cm Broadcast TV, ch. 14-83: 440- 884 MHz (UHF) Broadcast TV, ch 8-13: 174- 210 MHz (VHF-III) FM Radio: 87.5-107.9 MHz (VHF-II) Broadcast TV, ch. 2-7: 55-83 MHz (VHF-I) AM Radio: 520-1700 kHz
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The Electromagnetic Spectrum Ultraviolet = UV λ ~ 400 nm to ~ 10 nm f ~ 8x10 14 to 3x10 16 Hz Infrared=IR λ ~a few mm to ~700 nm f ~ 10 11 to 4x10 14 Hz
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The Electromagnetic Spectrum Gamma-rays: assoc. with nuclear decay X-rays: highest-energy atomic transitions
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Energy transport As the EM wave propagates, it will transport energy INTENSITY, I: rate at which energy is transported across a unit area (Power/Area = (Energy/Time) / Area)
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Plane wave: I does not change But if the origin is a point source, and emission is isotropic: wavefronts are expanding spheres. intensity moving through a given point in space:
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LECT19_21B - Maxwells Equations Predict Speed of EM Waves...

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