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# hw 38 - nguyen(jmn727 homework 38 Turner(59070 This...

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nguyen (jmn727) – homework 38 – Turner – (59070) 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 7 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: 4 . 6699 × 10 8 N. Explanation: Let : P = 7 mW = 0 . 007 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 . 007 W) 2 . 99792 × 10 8 m / s = 4 . 6699 × 10 8 N . 002 10.0 points At what distance from a 83 W electromag- netic wave point source (like a light bulb) is the amplitude of the electric field 20 V / m? μ o c = 376 . 991 Ω . Correct answer: 3 . 52846 m. Explanation: Let : P = 83 W , E max = 20 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 ) = (20 V / m) 2 2 (376 . 991 Ω) = 0 . 530516 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 = 83 W . Solving for the radius we get, R = radicalbigg P 4 π I = radicalBigg 83 W 4 π (0 . 530516 W / m 2 ) = 3 . 52846 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. Its wavelength remains unchanged, but its frequency increases. 3. Its frequency remains unchanged, but its wavelength decreases. correct 4. Its wavelength remains unchanged, but its frequency decreases.

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hw 38 - nguyen(jmn727 homework 38 Turner(59070 This...

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