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u
I
c
fI
c
==
+
()
,
1
0
the same as radiation pressure.
23. 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
0
2
r
f I
p
c
=
and the absorbed portion exerts a radiation pressure of
p
c
a
=
−
,
1
0
where
I
0
is the incident intensity. The factor 2 enters the first expression because the
momentum of the reflected portion is reversed. The total radiation pressure is the sum of
the two contributions:
00
0
total
2(
1
)
(
1
)
.
ra
f
If
I
f
I
pp
p
cc
+
−+
=+=
=
To relate the intensity and energy density, we consider a tube with length
A
and cross
sectional area
A
, lying with its axis along the propagation direction of an electromagnetic
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This note was uploaded on 06/03/2011 for the course PHY 2049 taught by Professor Any during the Spring '08 term at University of Florida.
 Spring '08
 Any
 Physics, Radiation

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