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hw21 - × 10-12 = 8 5 × 10 3 ±or the pulse to contain...

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Physics 21 Fall, 2004 Solution to HW-21 32 P 12 What is the frequency of a microwave of wave- length is 1.80 cm? = c f = c λ = 3 . 0 × 10 8 m / s 0 . 018 m = 1 . 67 × 10 10 Hz 32 P 18 Pulsed lasers used for science and medicine pro- duce very short bursts of electromagnetic energy. If the laser light wavelength is 1062 nm (this corresponds to a Neodymium-YAG laser), and the pulse lasts for 30 picosec- onds, how many wavelengths are found within the laser pulse? How short would the pulse need to be to fit only one wavelength? The pulse length L is the distance a wave crest travels in 30 ps: L = ct = (3 . 0 × 10 10 m / s)(30 × 10 - 12 s) = 9 × 10 - 3 m The number of wavelengths N in a pulse is N = L λ = 9 ×
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Unformatted text preview: × 10-12 = 8 . 5 × 10 3 ±or the pulse to contain only one wavelength, the pulse du-ration would have to be about N times shorter: 30 × 10-12 s 8 . 5 × 10 3 = 3 . 5 × 10-15 s = 3 . 5 fs 32 P 19 The E Feld in an EM wave has a peak of 36.5 mV/m. What is the average rate at which this wave carries energy across unit area per unit time? The energy ²ux is given by the magnitude of the Poynting vector: S = 1 2 ± cE 2 peak . Substituting the numbers yields S = 1 2 (8 . 85 × 10-12 )(3 . × 10 8 )( . 0365) 2 = 1 . 77 × 10-6 W / m 2 . November 2, 2004...
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