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27. Let
f
be the fraction of the incident beam intensity that is reﬂected. The fraction absorbed is 1
−
f
.
The reﬂected portion exerts a radiation pressure of
p
r
=
2
fI
0
c
and the absorbed portion exerts a radiation pressure of
p
a
=
(1
−
f
)
I
0
c
,
where
I
0
is the incident intensity. The factor 2 enters the Frst expression because the momentum of the
reﬂected portion is reversed. The total radiation pressure is the sum of the two contributions:
p
total
=
p
r
+
p
a
=
2
fI
0
+(1
−
f
)
I
0
c
=
(1 +
f
)
I
0
c
.
To relate the intensity and energy density, we consider a tube with length
`
and crosssectional area
A
, lying with its axis along the propagation direction of an electromagnetic wave. The electromagnetic
energy inside is
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 Fall '08
 SPRUNGER
 Physics, Radiation

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