Black-body Radiation
The experiment and apparatus
This experiment measures the intensity distribution as a function of wavelength for light
emitted by an incandescent lamp; in addition the temperature of the lamp is varied by
adjusting the applied voltage. The lamp filament is approximately a ‘black-body’
meaning it can absorb and emit electromagnetic radiation of all wavelengths.
Begin by examining the equipment; make sure that you are able to identify the
components shown in the figure below.
lamp or
‘black-body’
collimator
lens
angle sensor
prism
light sensor
power supply
measuring unit
Light emitted by the lamp is directed by the collimator lens to the prism. The light is then
dispersed by the prism, producing a diffracted light spectrum. The relative intensities of
the different wavelengths are measured by the light sensor that moves along a circular arc
in such a way that only a narrow range of wavelengths strikes the sensor at any one time.
The angle sensor measures the angle between the sensor and the prism. The computer
programme calculates the wavelength corresponding to this angle and produces a graph
of light intensity as a function of wavelength. The computer programme also guides the
power supply of the lamp, and is able to calculate the temperature of the lamp from the
electric power input.
Measurements
• Delete any data stored from previous measurements: Experiment > Delete ALL data
runs
• Set the power supply voltage on the control box to 7 V, turn the voltage on and check
that the light comes on
• Move the light sensor arm counter-clockwise until it reaches the stopper
• To calibrate the light sensor put your hand in front of the lamp, between the collimator
lens and the light sensor, and press the TARE button
• To start taking measurements press the START button
• Slowly turn the light sensor arm clockwise until the Light Intensity vs Wavelength
graph passes 5000 nm
• To stop taking measurement press the STOP button
• Adjust the signal generator voltage to 8 V
• Move the light sensor arm back to the stopper
• Calibrate the light sensor as before and take a new set of measurements
• Repeat the experiment again but with voltages of 9 V and 10 V
• When you have finished taking measurements, turn off the lamp by clicking OFF on the
signal generator control panel.
Avoid keeping the lamp voltage at 10 V for too long as this can blow the lamp and will
shorten the lamp life.
Findings and conclusions
How does the wavelength at the intensity peak compare to the wavelength of visible light
(350-700 nm) at all temperatures used in this experiment? What does this say about the
efficiency of the incandescent lamp as a light source?
What happens to the intensity peak as the filament temperature increases? How is this
related to the change in colour of the light?
In what way does the intensity graph obtained contradict the classical theory prediction of
electromagnetic radiation intensity distribution (consider the ultraviolet catastrophe)?