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University of TexasPHY 317LEM waves and particles-solutions

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This print-out should have 17 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points Consider the electromagnetic wave ... pattern as shown in the figure below. E B What is the direction in which the wave is traveling? 1. from-left-to-right. correct 2. undetermined, since the figure shows a standing wave. 3. from-right-to-left. Explanation: The direction of traveling is the direction of E~ × B~ ; i.e., from-left-to-right. 002 10.0 points A point light source delivers a power P. It radiates light isotropically (equally in all directions). A mirror of area A is placed at point B that is at distance r from the source. The mirror’s surface is perpendicular to the vector from the source to point B. Make the approximation that all the light hitting the mirror is perpendicular to it. The mirror is a totally reflecting surface. B r Find the total force on the mirror. 1. F = P c 2. F = 2 π r2 P c A2 3. F = A2 P 2 c π r2 4. F = A P c 2 π r2 correct 5. F = 4 π r2 P c A 6. F = P c 2 π r 7. F = 2 P c 8. F = π r2 P c 9. F = 2 P c π r2 10. F = 2 A P c r2 Explanation: Since the mirror is small enough (or far enough away) that we may approximate the incoming light as perpendicular, the force is simply F = p A , where p is the radiation pressure. The mirror is totally reflecting, so p = 2 I c . Intensity is defined by I = Power hitting the mirror Area of mirror A . I = P 4πr A 2 A = P 4 π r2 , leading to a force of F = p A = 2 I A c = A P c 2 π r2 on the mirror [Show More]

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