Optical Instruments
345
25.14
(a) When the child clearly sees objects at her far point
( )
max
125 cm
p
=
the lenscornea
combination has assumed a focal length suitable of forming the image on the retina
( )
2.00 cm
q
=
. The thin lens equation gives the optical power under these conditions
as
11
1
rs
1.25 m
0.020 0 m
in meters
fp
q
==
+
=+
50.8 diopte
=
+
far
P
When the eye is focused
( )
2.00 cm
q
=
on objects at her near point
( )
min
p
10.0 cm
=
the optical power of the lenscornea combination is
1
m
0.020 0
r
in meters
+
1
0.100
q
+
=
1
60.0 diop
m
=
+
ters
nea
P
(b) If the child is to see very distant objects
( )
p
→∞
clearly, her eyeglass lens must
form an erect virtual image at the far point of her eye
( )
125 cm
q
=−
. The optical
power of the required lens is
1
0
800
1.25
in meters
q
+
=
+
−
−
P
1
0.
m
=
diopters
Since the power, and hence the focal length, of this lens is negative, it is
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This document was uploaded on 10/20/2011.
 Spring '09
 Physics, Work

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