PHYS 434
Optics
Problem set #2
1. A convenient way to measure the focal length of a positive lens make use of the following
fact.
If a pair of points which satisfy the image–object relation are separated by a
distance
L >
4
f
, there will be two locations of the lens, a distance
d
apart, for which
this pair of points still image. Show that
f
=
L
2

d
2
4
L
.
Note that this avoids measurements made specifically from the vertex, which are gen
erally not easy to do.
2.
Do you really see things upside down?
Here is way you can convince yourself
that the image on your retina is inverted.
Look through a pinhole at a broad light
source.
Look at a pencil point through the hole and look at its shadow on your
retina.
Everything behaves as expected.
Now reverse the order and put the pencil
point between the pinhole and your eye. Move the pencil and notice the direction of
motion of the shadow! Describe what you see. Also make a sketch and explain what’s
happening.
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 Winter '09
 Sutton
 Parabolic reflector, spherical aberration, parabolic mirror

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