Physics 7C, Spring 2004, Section 1, Instructor: Prof. Adrian Lee First Midterm Examination, Monday February 23, 2004 Please do work in your bluebooks. You may use one double-sided 3.5" x 5" index card of notes. Test duration is 120 minutes. la) (20 points total) Consider the generation of an electromagnetic wave from a 1/4 wavelength long dipole antenna. In the dipole, electrons move up and down sinusoidally. For several phases of the varying current, sketch the E and B fields from the dipole antenna and indicate which way the current is flowing. b) In the same sketch, show two launched "bubbles" of electromagnetic waves and indicate both the direction of propagation and the direction of the transverse E and B fields. 2a) (40 points total) Show that a thin converging lens of focal length f followed by a thin diverging lens of focal length -f will bring parallel light to a focus beyond the second lens provided that the separation of the lenses L satisfies 0 < L < f. (20 points) b) Does this property change if the lenses are interchanged? (10 points)
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