Physics Experiment (Experiment 2: Blackbody Radiation)
ISO Training: Phase 3 Experimental Session (Physics Experiment)
Experiment 2: Blackbody Radiation（黑體輻射）
Purpose of this lab
 To study the properties of the blackbody radiation and verify the Wien’s radiation law
（維恩輻射定律）.
Objective of this experiment

hc
, where h is the Planck constant（普朗克常數）, c is the
k
speed of light in vacuum and k is the Boltzmann constant（波爾茲曼常數）.
To determine the value of
Theory
A blackbody radiator（黑體輻射體）consists of a block of material having an internal
cavity connected to the outside surface of the block by a small hole. The radiation emerging
from the hole is called blackbody radiation 1 . Its characteristic depends only on the
temperature T, measured in absolute temperature（絕對溫度）2, of the cavity walls and not
at all on the shape of the cavity or on the material forming the cavity walls.
6
R
5
4
3
2
1
0
0
10000
20000
30000
Wavelength (nm)
Fig. 1 Spectral radiancy of a blackbody at 500 K
Fig. 1 shows how the radiation emitted by a blackbody is distributed in wavelength（波長）.
The quantity R , plotted on the vertical axis in Fig. 1, is called the spectral radiancy（頻譜
光度）. It is defined so that R d is the rate at which energy is radiated per unit area of
surface for wavelengths lying in the interval  to   d . The relation between R ,  and
T is given by the Planck's radiation law（普朗克輻射定律）,
1
The cavity can be discussed as an absorber as well as an emitter of light. If the cavity block is held at room
temperature and viewed by ambient light the small hole that penetrates to its interior appears black. Light that
enters this hole is trapped within the cavity, which behaves like a perfect absorber of the incident. It is on this
basis that cavity radiation is called blackbody radiation.
2
Absolute Temperature in Kelvin (K) = Temperature in Celsius (C) + 273.15, e.g. 6,000K = 5,726.85C.
Physics Experiment (Experiment 2: Blackbody Radiation)
R 
8 hc
.
 (e hc / kT  1)
5
(1)
When the values of temperature T and the wavelength  are small, Equation (1) can be
approximated by the Wien's radiation law,
R 
8 hc

5
e  hc / kT .
(2)
In this experiment we shall use a tungsten filament as the blackbody radiator to study the
hc
spectral radiancy R as a function of temperature T and determine the value of
. The
k
temperature of the tungsten filament（鎢絲）is estimated by the equation:
1.2
R
 T 

 ,
R298  298 
where T is the temperature and R298 is the resistance（電阻）at room temperature (298 K).
Apparatus list






Monochromator（單色儀）
Photomultiplier tube（光電倍增器）(Fig 2)
Tungsten light bulb（鎢絲燈泡）
1 k resistor（電阻器）
Power supplies
Digital multimeters（數字電表）
Fig 2 Schematic of a photomultiplier tube (PMT). The output current of PMT is directly
proportional to the number of incident photons（光子）- radiancy.
(Wikimedia Commons)
(3)
Physics Experiment (Experiment 2: Blackbody Radiation)
Procedures
Monochromator
PMT
Light
Bulb
Power
Supply
Ammeter
Power
Supply
A
V
Fig. 3 Experimental setup
1. Set up experiment as shown in Fig. 3 and set the monochromator to 350 nm.
2. Adjust the input voltage of the PMT to 12 Volt.
3. Adjust the voltage of the power supply such that the voltage across the light bulb is 7 Volt.
4. Adjust the position of the tungsten lamp such that the output current of the PMT is about
100 A.
5. Record the voltage V and current I across the tungsten lamp and the output current Ip of
the PMT.
6. Vary the voltage V across the tungsten lamp and repeat step 5.
7. Disconnect the current to the tungsten lamp and record the background current IB of the
PMT.
8. Add the 1 k resistor in series with the tungsten lamp. Adjust the current to 1 mA and
record the voltage across the tungsten lamp. Calculate the resistance R298 of the tungsten
lamp at room temperature.
9. Fill in the table and plot ln  I p  I B  against
10. Determine the value of
hc
from the slope.
k
1
.
T
Physics Experiment (Experiment 2: Blackbody Radiation)
Safety Precautions (General)

Always turn off the power before moving, installing and inspecting the apparatus or
connecting/disconnecting the cables and connectors.

Do not modify any part of the apparatus and do not open the housing case (if any).
Malfunctions or electrical shocks might result and the apparatus might overheat, smoke
or catch fire.
Precautions when Handling PMT Modules

Do not expose the photocathode of PMT modules to excessive light such as sunlight. If
exposed, noise will increase, photocathode sensitivity will deteriorate and the PMT will
even be damaged.

Do not touch the light input window with bare hands. If the window becomes soiled
with dirt or grime, wipe it clean using alcohol.

Carefully check that the power supply output voltage and polarity are correct.

Do not apply strong vibrations or impacts to PMT modules.

Do not apply a strong tightening force to localized sections.

Do not let moisture or dust penetrate inside.
Physics Experiment (Experiment 2: Blackbody Radiation)
Data Sheet
Name: _________________________
Date: _________________________
Pre-lab Questions
1. What is the expected value of
hc
?
k
________
2. What is the expected value for the slope at step 9?
________
Radiancy as a function of temperature
V
(V)
I
(A)
Ip
(mA/A)
7
6.5
6
5.5
5
4.5
4


Background current of the PMT IB
R
()
=
_
 R 

T  
R
 298 
(K)
1
. Slope =
T

298
1
T
ln( I p  I B )
__ mA
Current through tungsten lamp at room temperature R298
Voltage across tungsten lamp at room temperature R298
Resistance of tungsten lamp at room temperature R298
Plot ln  I p  I B  against
1 / 1. 2
__
=
__ 
= __ __ 
= ___ __ 
____ mA
____ mV
____ 