In-class problems 4 Solutions

# In-class problems 4 Solutions - 2(2(c At this 7.20 m...

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Name Student ID Section In-Class Problem, Week 7 13 May 2008 Show ALL your work, Total Points Possible = 8 1. Is it possible to have total internal reﬂection for light incident from air on water? Explain. (2 pts) No, light moving from air ( n = 1) to water ( n = 1 . 33) cannot undergo total internal reﬂection, since total internal reﬂection only occurs when light moves from a medium of high index of refraction to a medium of low index of refraction. +1 for correct answer, +1 for explanation 2. A spherical lightbulb of diameter 3.20 cm radiates light equally in all directions, with power 4.50 W. (a) Find the light intensity at the surface of the lightbulb. (+ 1pt) I = P 4 πr 2 = 4 . 50 W 4 π (1 . 60 × 10 - 2 m ) 2 = 1 . 40 kW/m 2 (1) (b) Find the light intensity 7.20 m away from the center of the lightbulb. (+ 1pt) I = P 4 πr 2 = 4 . 50 W 4 π (7 . 20 m ) 2 = 6 . 91 mW/m
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Unformatted text preview: 2 (2) (c) At this 7.20 m distance, a lens is set up with its axis pointing toward the lightbulb. The lens has a circular face with a diameter of 15.0 cm and has a focal length of 35.0 cm. Find the diameter of the bulb. (+ 2pts) (+ 1pt) 1 p + 1 q = 1 f ⇒ 1 7 . 20 m + 1 q = 1 . 350 m ⇒ q = 0 . 386 m (3) (+ 1pt) M = h h =-q p ⇒ h 3 . 20 cm =-. 386 m 7 . 20 m ⇒ h = 0 . 164 cm (4) (d) Find the light intensity at the image. (+ 2pts) Lens intercepts power given by: (+ 1pt) P = I bulb A = (6 . 91 × 10-3 W/m 2 )( π 4 (0 . 150 m 2 ) = 1 . 22 × 10-4 W (5) and puts it all onto the image with intensity (+ 1pt) I image = P A = 1 . 22 × 10-4 W π (0 . 00164 m ) / 4 = 57 . 8 W/m 2 (6) Some possibly useful equations: 1...
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## This note was uploaded on 09/08/2008 for the course PHYS 3B taught by Professor Wu during the Spring '08 term at UC Irvine.

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