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4.4 Blackbody Radiation

By the end of this lesson you can:

  • Describe a blackbody and its spectrum
  • Relate temperature to peak wavelength
  • Explain how radiated power depends on temperature
Find your course: NSW Year 11NSW Year 12: Module 7, IQ3QLD Unit 4: Quantum theoryVIC Unit 1: Light and heatAP Physics 2: Unit 15IB: B.1 Thermal energy transfersIB: B.2 Greenhouse effect
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Learn the idea: a short explanation, then a check.
In the latter half of the 19th century scientists studied light coming off heated black bodies, and discovered, when they graph the intensity of each wavelength with the wavelength the graph had a very characteristic shape. This became known as the black body curve. As scientists examined the energy coming off black bodies experimentally, they established some relationships of the curve with temperature.
Theory
​Wilhelm Wien established a relationship between the wavelength with  the greatest intensity with the temperature of the emitting body.

This video examines the basics of the blackbody curve and its relation to the temperature of a radiation emitting blackbody, as described by Wien's Displacement Law.
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Test your predictions in a simulation.

Examine this interactive from pHET. Change the temperature of the emitting body and see how this affects both the area and the positoin of the peak.
What do you notice?

Going Deeper


Numerous scientists in the late 1800s were interested in the nature of the radiation coming off black bodies and the temperature of those black bodies.
Apart from Wien, who is established the Displacement Law (as previously discussed), two other scientists, Josef Stefan and Ludwig Boltzmann, established a relationship between the total intensity of radiation of a black body with its temperature. This became Stefan-Boltzmann Law.

What was missing at the time was a mathematical basis for the blackbody curve. This eventually led Max Planck to postulate light that the energy of a black body is given off discreetly that is in fixed amounts. This was in contrast to the belief that energy needed to be emitted continuously. Nonetheless his postulate that allowed him to develop a mathematical model to explain the shape of the curve and led eventually to a quantum understanding of nature. This is further discussed in this lesson.