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HW 38 - terry(ect328 homework 38 Turner(59130 This...

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terry (ect328) – homework 38 – Turner – (59130) 1 This print-out should have 10 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A 1 mW laser beam is reflected back upon itself by a mirror. Calculate the force on the mirror. The speed of light is 2 . 99792 × 10 8 m / s. Correct answer: 6 . 67128 × 10 9 N. Explanation: Let : P = 1 mW = 0 . 001 W and c = 2 . 99792 × 10 8 m / s . The intensity of the laser is P A with P the power and A the area of the beam. The average momentum per unit area per unit time (force per unit area) transferred to the wall is Δ p Δ t A = F A = 2 I c = 2 P A c , so F = 2 P c = 2 (0 . 001 W) 2 . 99792 × 10 8 m / s = 6 . 67128 × 10 9 N . 002 10.0 points At what distance from a 68 W electromag- netic wave point source (like a light bulb) is the amplitude of the electric field 22 V / m? μ o c = 376 . 991 Ω . Correct answer: 2 . 9034 m. Explanation: Let : P = 68 W , E max = 22 V / m , and μ 0 c = 376 . 991 Ω . The wave intensity (power per unit area) is given by I = S av = E 2 max 2 ( μ o c ) = (22 V / m) 2 2 (376 . 991 Ω) = 0 . 641925 W / m 2 . The constant μ o c (impedance of free space) turns out to be 120 π Ω. You should verify that (V / m) 2 Ω = W / m 2 . With this we can find the intensity, which multiplied by the area of a circle of radius R must gives us the source’s power P , therefore 4 π R 2 I = P = 68 W . Solving for the radius we get, R = radicalbigg P 4 π I = radicalBigg 68 W 4 π (0 . 641925 W / m 2 ) = 2 . 9034 m . 003 10.0 points A light wave passes from air into water. Which of the following is true? 1. Its frequency remains unchanged, but its wavelength increases. 2. None of these. 3. Both its wavelength and its frequency remain unchanged. 4. Its wavelength remains unchanged, but its frequency decreases.
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