Black Body Radiation - Understanding the black body spectra using classical and quantum physics

When a black body has a temperature, it emits light. This light is called black body radiation and it has a spectra (the intensity of the light depends on its wavelength.)

This is why hot metal appears red, or even orange. Another example: the colour that we perceive of a star allows us to determine its surface temperature.

In this video you will learn how to read a black body spectra. At the same time, we will play around with the black body spectra of the sun and answer questions like "what is the real colour of the sun?", "why does the sun appears yellow to us, then orange/red in the evening ?" and even "Why the sky is blue?".

And then, we will dive into the physics of Black Body Radiation: together, we’ll realise that classical physics cannot explain the shape of the black body spectra. Worse, it predicts that heating any object, even mildly, would flood the surroundings with deadly radiation, which of course does not happen. This is called the Ultraviolet Catastrophe.

We will see that quantum mechanics comes to the rescue and leads to Plank’s Law which models very well a black body spectra. The integration of Plank’s Law leads to Stefan Boltzmann’s law, and its differentiation to Wiens displacement law. Wiens displacement law allows to predict the colour of the light emitted by a body when knowing its surface temperature.

We will conclude this video with a direct application of Wien’s displacement law.

Content of the video: 0:00 What happens when a black body is heated? 0:56 Why does a black body emit light? (Summary of the 1st episode). 2:00 Origin of the black body spectra. 2:32 Reading a black body spectra. 3:41 What is real colour of the Sun? 4:16 Why does the sun appear yellow? 4:30 Why is the sky blue? 5:06 Why does is hot metal red ? 5:29 The shape of the black body radiation spectra. 6:20 The Ultraviolet Catastrophe: understanding black body radiation with classical physics (Rayleigh-Jeans Law). 7:13 Plank’s Law: understanding Black Body radiation with quantum mechanics. 7:43 Wiens displacement law: derivation and determining the colour of a black body from its temperature. 7:58 Wiens displacement law: application exercise.

Credits: Space background: Traverse galaxy Background: by F. Summers, G. Bacon (STScI). Simulation by C Hummels, P. Hopkins (CalTech) and A. Wetzel (UC Davis)

Infrared picture of the Lemur By Arno / Coen CC BY-SA 3.0,

All other illustrations not credited here on in the video are by Edouard Reny.

Music by Edouard Reny.

This video is produced and presented by Edouard Reny, Ph.D. in solid state chemistry and private tutor in Physical Sciences.

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