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Unformatted text preview: 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 p tA = F A = 2 I c = 2 P Ac , 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? o c = 376 . 991 . Correct answer: 3 . 52846 m. Explanation: Let : P = 83 W , E max = 20 V / m , and c = 376 . 991 . The wave intensity (power per unit area) is given by I = S av = E 2 max 2 ( o c ) = (20 V / m) 2 2 (376 . 991 ) = 0 . 530516 W / m 2 . The constant o c (impedance of free space) turns out to be 120 . You should verify that (V / m) 2 = 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 sources power P , therefore 4 R 2 I = P = 83 W . Solving for the radius we get, R = radicalbigg P 4 I = radicalBigg 83 W 4 (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....
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This note was uploaded on 01/21/2010 for the course PHY 303L taught by Professor Turner during the Spring '08 term at University of Texas at Austin.
- Spring '08