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PROBLEM 21 &amp; 22 April 23 2010

# PROBLEM 21 &amp; 22 April 23 2010 - mirror 5 times...

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Solving Problems 21 & 22 April 21, 2010 Problem 21 (like 25.3): A scientist wishes to make it easier to measure light pressure by reflecting a laser beam between the two mirrors of a pair of radiometers. What is the greatest number of times a 1 mm wide light beam can reflect from one of a pair of parallel mirrors 10 mm in width? a) 1; b) 3; c) 5; d) 7; e) 10. What do you know? The law of reflection dictates that the minimum spacing between reflections on one of the mirrors is the same as the width of the beam, else some of the incident beam will be cut off by missing the mirror. So … there is 2 mm per bounce and thus the beam hits each
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Unformatted text preview: mirror 5 times. Problem 22 (like 25.22): A laser beam for a barcode reader is shining on a mirror that rotates 100 times per second. What is the velocity of the laser spot on a cabbage that is 0.1 m from the axis of the mirror? a) 63 m/s; b) 126 m/s; c) 252 m/s; d) 504 m/s; e) 100 m/s. What do you know? The rotating mirror actually makes the light beam sweep across the cabbage twice per full rotation. So … The laser spot is going around with an angular frequency of 200 times two pi per second = 400 pi per sec, although it only covers a 180 degree sweep. The velocity at a radius of 0.1 m is r times omega or 40 times pi m/s = 126 m/s....
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