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Unformatted text preview: Physics 7C, Fall 2003, Section 1
Instructor: Prof. Adrian Lee First Midterm Examination, Monday Sept 29 Please do work in your bluebooks. You may use one double sided 3 5”); 5” index card of notes. Test duration 18 90 minutes “'— [digpi
l&" 5. 1) An uptight object is placed a distance in front of a converging lens equal to twice the
focal length f1 of the lens. On the other side of the lens is a converging mirror of focal
length f2 separated from the lens by distance 2(f1 + f2) (40 points total) a) Make a ray diagram showing where the image is for a person looking from the object
side of the lens. (10 points) b) What is the distance from the lens to the image? (15 points) C) What IS the magnification (including orientation) of the 1mage in terms of the object , height h? (10 points) d) Is it a real or virtual image? (5 points) 2) For the human eye, the near—point distance is the closest distance from the eye that the
eye can focus on. Similarly, the far—point distance is the furthest point the eye can focus
on. A nearsighted person has a farpoint distance of 15 cm and a nearpoint distance of
10 cm. In all parts, disregard the eyetolens distance. (40 points total) a) Give the sign and magnitude of the focal length of the lens needed to correct this far
vision. Note that after passing through this lens, light from infinity should appear to
come from 15 cm. Is the lens convex or concave? A ray drawing may help your
thinking. (20 points) b) With this lens in place, what is the distance to this person's near point?
(20 points) 3) Consider twoslit interference where the slits are identical, spaced by a distance D and
illuminated by coherent light. The distance from slits to screen is L. (40 points total) a) If you move the slits further apart to 2 x D how do you have to move the screen to
have the position of the peaks and valleys remain unchanged? Assume that you are in the
small angle limit. (12 points) b) Calculate the intensity as a function of angle either algebraically or by the phasor method in terms of Io the intensity at the center of the pattern. The following relation
may be useful. (16 points) Sin(A) + Sin(B) = 25in(A : B Jays“ ’ B ] 2 c) If a slab of clear dielectric with thickness t and index n is put over one slit, how does the intensity pattern change? Is there a relation between n, t, and A to have no change of
the pattern? (12 points) . .4) It is possible to levitate a spherical glass bead in a vertically upward directed laser
beam. If the bead mass In is 2 x 1045 gm, and its density r = 0.3‘ gm/cm3, what power, P,
laser beam will support the bead? Assume the laser beam diameter d = 2 mm and that the
bead reﬂects the laser light. For simplicity, assume the light is all reﬂected directly
downwards. (20 points total) ...
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 Fall '08
 LIN
 Physics

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