EE119 Discussion Section 3
(02/08/10)
Professor: Jeff Bokor
TA: Xi Luo
1.
More on Lenses
(1) Uncle QK finds a lens on his way to the lab and wants to determine the focal
length of the lens. He constructs an optical axis between a fixed object and a fixed
screen, at a distance L apart (note: L is at least four times the focal length of the
lens).
He notices two positions of the lens that focus an image of the object on
the screen, magnified in one case and reduced in the other.
a)
If the two lens positions differ by a distance D, prove to Uncle QK that he can
determine the focal length of the lens by f = (L
2
– D
2
)/4L. (This is known as
Bessel’s method
for finding the focal length of a lens)
b)
Why does L have to be at least four times the focal length of the lens for
Bessel’s method to work?
(i.e. prove that the minimum separation between
conjugate real object and image points for a thin positive lens is 4f)
(2) For the combination of one prism and 2 lenses shown (Fig. 1), find the location
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 Spring '09

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