Chapter 36
347
Q36.8
We assume the lens has a refractive index higher than its surroundings. For the biconvex lens in
Figure 36.27(a),
R
1
0
>
and
R
2
0
<
. Then all terms in
n
RR
−−
F
H
G
I
K
J
1
11
12
af
are positive and
f
>
0. For the
other two lenses in part (a) of the figure,
R
1
and
R
2
are both positive but
R
1
is less than
R
2
. Then
>
and the focal length is again positive.
For the biconcave lens and the planoconcave lens in Figure 36.27(b),
R
1
0
<
and
R
2
0
>
. Then
both terms are negative in
−
and the focal length is negative. For the middle lens in part (b)
of the figure,
R
1
and
R
2
are both positive but
R
1
is greater than
R
2
. Then
<
and the focal
length is again negative.
Q36.9
Both words are inverted. However OXIDE has updown symmetry whereas LEAD does not.
Q36.10
An infinite number. In general, an infinite number
of rays leave each point of any object and travel in
all directions. Note that the three principal rays
that we use for imaging are just a subset of the
infinite number of rays. All three principal rays can
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 Fall '11
 Staff
 Physics, Geometrical optics, infinite number, biconvex lens

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