101
I.3
In this experiment, a tungsten filament lamp is taken to be a black body radiator.
Using a monochromatic filter, radiation with frequency in the visible region is
selected. For the range of temperatures of the tungsten filament, the energy density
can be taken to be given by eq. (2). The energy density at the chosen frequency is
indirectly measured by measuring the photocurrent I
ph
generated upon exposing a
photocell to the radiation. From the properties of the photoelectric effect, it is known
that the photocurrent is proportional to the intensity of the radiation. Thus
kT
h
3
3
ph
e
c
h
8
)
(
U
I
ν

ν
π
≈
ν
∝
 (3)
or
t
tan
cons
kT
h
c
h
8
ln
kT
h
I
ln
3
3
ph
+
ν

=
ν
π
+
ν

=
(4)
Hence the graph of ln I
ph
Vs 1/T will be a straight line of slope of magnitude
h
°
/K.
I.4
The temperature of the tungsten filament can be varied by changing the current
through it. The temperature of the filament can be estimated by measuring the
resistance R of the filament. The variation of R with temperature for tungsten is
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 Fall '09
 LIND
 Energy, Light, Energy density, calibration graph, Tungsten filament

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