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
This is the end of the preview.
Sign up
to
access the rest of the document.
 Spring '09
 Physics, Work

Click to edit the document details