What Are Wien's Law & the Stefan-Boltzmann Law?

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  • 0:02 Why Does an Object Glow?
  • 1:12 Blackbody Radiation &…
  • 2:37 Why Does Hotter = Brighter?
  • 3:24 Wien's Law and the…
  • 6:00 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will explain why an object glows and why it changes color with temperature as we take a look at blackbodies, electromagnetic radiation, Wien's Law, and the Stefan-Boltzmann Law.

Why Does an Object Glow?

Did you ever wonder why and how a hot object emits light? Maybe you've seen how a blacksmith takes out a piece of metal, like a metal rod from a fire, only to have it glow this rich red color. You see a color because when a hot object glows, it actually emits photons, packets of electromagnetic radiation, including visible light.

A hot object, like a person who is fuming mad at something, is really agitated. That agitation produces collisions and changes in motion between particles, including electrons, of the object in question. This causes energy to be carried away from the object as electromagnetic radiation. In essence, the really hot object is 'letting off steam,' albeit in the form of light, in order to cool off a bit, like a person may need to let off some steam when they're agitated.

How an object's temperature influences its beautiful glow goes way beyond this little tidbit. You'll soon learn why objects get brighter as they get hotter and why they change colors with temperature as we take a look at Wien's Law and the Stefan-Boltzmann Law.

Blackbody Radiation & Temperature

A theoretical object that can be perfectly efficient at absorbing and emitting radiation is known as a blackbody, and it therefore emits what's known as blackbody radiation.

A blackbody is black at room temperature, hence the name. However, at higher temperatures, it can actually glow at visible wavelengths. Therefore, you should be aware that astronomers and physicists use the term 'blackbody' to also refer to objects that glow.

Naturally, given what we've went over so far, you may tend to think that blackbody radiation is given off only by hot objects like that hot metal rod or an extremely hot star. Not so. Blackbody radiation is the radiation emitted by a heated object. But 'heated' is a relative term. What I mean is, it's heated when compared to absolute zero - 0 Kelvin, -273.2 Celsius or -459.7 Fahrenheit.

Knowing this, it won't surprise you to learn that blackbody radiation is emitted by everything from stars and light bulbs to ice cubes. That's because at temperatures above absolute zero, particles in an object still have thermal energy and thus radiate photons. An ice cube may be as cool as a cucumber but compared to absolute zero, it's as agitated and hot as you are when your computer freezes or car doesn't start.

Why Does Hotter = Brighter?

Ok, so now you know why stuff glows. Now we need to learn how the temperature and the color of the glow of an object interact. Surely you've seen how as metal is heated more and more, it actually changes the color of light it emits from red, to orange, to yellow, as it gets hotter. Now you'll learn why it changes color and gets brighter as it gets hotter, not just why it glows.

Firstly, the hotter an object is, the more blackbody radiation is emitted by it. Why? Well, the madder you are, the more agitated you are, and the more steam you have to blow off. The hotter an object, the more collisions there are, the more violent those collisions, and the more radiation is emitted. This is why as things get hotter, they get brighter.

Wien's Law and the Stefan-Boltzmann Law

Check out the graphs below representing blackbody curves to see this concept represented visually. Note that these graphs have a y-axis indicating intensity and an x-axis representing wavelength.

Blackbody curves
Graph of blackbody curves

You see larger areas under curves representing hotter objects, where the area under the curve is proportional to the total energy emitted. So, a hotter object emits more radiation (including visible light). This concept refers to the Stefan-Boltzmann Law, which says, in simple terms, that the total energy radiated from a blackbody is proportional to the fourth power of its temperature.

Secondly, the wavelength of photons an object emits is dependent on the temperature of the object. Most electrons in an object travel at intermediate speeds while a few travel really fast or really slow, just like most people travel at average speeds on the highway and only a few crazy or careful ones drive faster or slower.

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