We can apply the paraxial approximation (Equation 2.4) to nd the length VQ:
RQC
QC
h
R
RQCVQ
h
R
=+
=+
−==
∆
3
2
2
2
2
Substituting for VQ then gives
n
s
n
s
nn
R
1
1
2
2
21
+=
−
()
Consider what happens when the distance s
1
becomes very large, that is, for an object at a very long
distance. In the limit that s
1
→∞, we have
n
s
nn
R
s
nR
nn
f
2
2
21
2
2
21
=
−
()
→=
−
()
='
Here, the light comes to a focus at a set distance f′ into the media with index n
2
. is is dened as the
focal point within medium 2. Since the light is coming from innity, the light rays would be parallel and
the wavefronts would be plane waves. If s
2
→∞, we have
n
s
nn
R
s
nR
nn
f
1
1
21
1
1
2
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