PHYSICS 7C Midterm: physics7C-fa2011-mt2-Aganagic-soln
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1. (a) (3 points) looking at figure 1, we see that sin = d = = d sin . Figure 1: figure for problem 1(a) (b) (3 points) the path length di erence = d sin leads to a phase di erence (between neighboring antennae) of = k = 2 . Thus we are interfering 4 waves, and the total electric eld is given by. Etot = e0 cos( t) + e0 cos( t + ) + e0 cos( t + 2 ) + e0 cos( t + 3 ) The corresponding phasor diagram is shown in figure 2 (e0 and etot in the gure label the lengths of the phasors). Figure 2: figure for problem 1(b) (c) (2 points) = 0 = = 0. D sin = d sin = /4 = = sin 1( . Figure 4: figure for problem 1(d) (b) (5 points) i. Ab = (3 108m/s 1. 5 10 8s)2 = 20. 25m2 ( s)2.
