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15.
(a) We assume all the power results in photon production at the wavelength
λ
= 589 nm. Let
R
be the
rate of photon production and
E
be the energy of a single photon. Then,
P
=
RE
=
Rhc/λ
,where
E
=
hf
and
f
=
c/λ
are used. Here
h
is the Planck constant,
f
is the frequency of the emitted
light, and
λ
is its wavelength. Thus,
R
=
λP
hc
=
(589
×
10
−
9
m)(100 W)
(6
.
63
×
10
−
34
J
·
s)(3
.
00
×
10
8
m
/
s)
=2
.
96
×
10
20
photon
/
s
.
(b) Let
I
be the photon ﬂux a distance
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 Fall '08
 SPRUNGER
 Physics, Energy, Power, Photon

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