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75. Let
θ
be the angle of incidence and
2
be the angle of refraction at the left face of the
plate. Let
n
be the index of refraction of the glass. Then, the law of refraction yields sin
=
n
sin
2
. The angle of incidence at the right face is also
2
. If
3
is the angle of
emergence there, then
n
sin
2
= sin
3
. Thus sin
3
= sin
and
3
=
.
The emerging ray is parallel to the incident ray. We wish to derive an expression for
x
in
terms of
. If
D
is the length of the ray in the glass, then
D
cos
2
=
t
and
D = t
/cos
2
.
The angle
α
in the diagram equals
–
2
and
x = D
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 Spring '08
 Any
 Physics

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