Last updated in Dec 2017 ©NISER
Page 3
reaching the anode. The voltage required to do this is called the “stopping potential" (
V
0
). A typical I-
V characteristics for a given frequency of light is also depicted in Fig. 2.
Thus Eq. 1 can be rewritten as,
Φ
−
=
e
eV
ν
h
0
… (2)
Φ
−
=
ν
e
h
0
V
… (3)
It is worth noting here that, since the anode and cathode surfaces are different, an additional
contact potential “A” comes into the picture which simply gets
added to the work function
“
Φ”. Eq. (3) can be written in terms of wavelength as
(
)
A
)
1
)(
e
hc
(
0
V
+
Φ
−
=
λ
… (4)
Standard value of
h is known
1
to be 6.626×10
-34
J-s.
Experimental Set up:
The present experimental set-up (see Fig. 3) comprises of a tungsten-halogen light source
with
five different colour filters, a Cesium-type vacuum phototube,
a built-in power supply
and a current multiplier. The base of the phototube is built into a dark room and in front of it
a receptor (pipe) is installed to mount filters.
Tungsten-Halogen lamp
Cs-type vacuum phototube
Figure 3: Experimental set up
Receptor
Last updated in Dec 2017 ©NISER
Page 4
Procedure:
1. Plug in and switch on the apparatus using the red button at the bottom right corner of
the set up.
2. Before the lamp is switched on, put the toggle switch in current mode and check that
the dark current is zero.
3. Turn on the lamp source (it may take 5-10 mins. to warm up). Set the light intensity
near to maximum. Note that the intensity should be such
that the value of current
should not exceed the display range. In case it happens,
you need to reduce the
intensity. You should not change intensity while taking data.
4. Insert one of the five specified filters into the drawtube of the receptor.
5. Set the voltage direction switch to “+ve” polarity. Adjust the voltage knob at
minimum and current knob at “X 0.1” position which means
the resolution is up to
one decimal point. Vary the voltage and record the current till the value of current
becomes relatively constant. Use the display mode switch
to record the values of
voltage and the corresponding current each time
6. Now, set the voltage direction switch to “-ve” polarity. Adjust the voltage knob at
minimum and current knob at “X 0.001” (we need higher resolution since current will
be less here). Vary the voltage and record the current till the value of current becomes
0. Use the display mode switch to record the values of voltage and the corresponding
current each time.
7. The above steps 5 and 6 provides data to plot the I-V characteristics of the phototube
for the wavelength (or frequency) selected by the filter.
8. Repeat the steps 5-7 for all the filters provided.
9. Fill up the observation tables and draw necessary plots.
Determine the values of
planck’s constant and work function of the metal used in the phototube.
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