# Theory question - Blackbody Radiation and Light

• sweetreason
However, the distribution of the photons may change according to Planck's law, resulting in a different intensity of light at different wavelengths. In summary, in thermal equilibrium, the number of photons emitted from a heated cavity remains constant, but the distribution of their energies may change according to Planck's law, resulting in a different intensity of light at different wavelengths.
sweetreason
"Theory" question -- Blackbody Radiation and Light

I am trying to understand the discussion about blackbody radiation in my Modern Physics textbook. (I'll quote it, but it can be found http://phy240.ahepl.org/Chp3-QT-of-Light-Serway.pdf" , page 5 document numbering, 69 textbook numbering).

The book says "light emitted by a small opening [in a heated cavity] is in thermal equilibrium with the walls, because it has been absorbed and re-emitted many times." (the book shows a diagram in which the light has bounced around a bit inside the cavity)

Now, I'm assuming the way that light exchanges heat has to do with the physics 1 equation $\frac{1}{2}mv^2_{av} = \frac{3}{2}k_BT$ (though I am not sure if this equation needs to be modified with a relativistic one). In other words, light has kinetic energy, and when it shines on you the light bounces into your molecules, which moves them around and thus heats them up. By conservation of energy and momentum, though, light would have to give up kinetic energy in order to heat you up. So once the light has reached thermal equilibrium, it must have given up a fair bit of kinetic energy. So how is the light that is in thermal equilibrium with the cavity changed when it exits? Light has no mass, it's energy is purely kinetic. I remembered reading somewhere that light that stopped moving essentially ceases to exist. Is that true? So would the number of photons leaving the cavity be less? Would it follow that the wavelength of the light decreases in accordance with Planck's law? Another bit of confusion here for me arises because of the connection between kinetic energy and velocity. If light gives up kinetic energy, you would at least classically expect it to give up velocity. Does this happen here? The speed of light in materials can be less than c, so this is allowable, right?

Last edited by a moderator:
The main question I'm asking is how does the light emitted from a heated cavity that has reached thermal equilibrium differ from the light coming in? Is there a decrease in number of photons and thus wavelength or a decrease in velocity, or something else? The key thing to remember when trying to answer this question is that in thermal equilibrium, the energy of the light must remain constant. This means that the number of photons leaving the cavity must be equal to the number of photons entering the cavity. So, while the photon energy may increase due to the increased temperature, the number of photons remains the same. Therefore, there is no change in wavelength or velocity of the light emitted from the cavity.

## Related to Theory question - Blackbody Radiation and Light

Blackbody radiation is the thermal electromagnetic radiation emitted by a perfect absorber and emitter of energy, known as a blackbody. It is based on the principle that all objects at a non-zero temperature emit radiation.

## What is the relationship between blackbody radiation and light?

Blackbody radiation and light are closely related as both are forms of electromagnetic radiation. Blackbody radiation refers to the thermal radiation emitted by an object, while light refers to the visible portion of the electromagnetic spectrum.

## What is the significance of blackbody radiation in physics?

Blackbody radiation is significant in physics as it helped to establish the field of quantum mechanics. The study of blackbody radiation led to the development of Planck's law, which explained the relationship between the wavelength of light and its energy.

## How does temperature affect blackbody radiation?

The temperature of an object directly affects the amount and wavelength of blackbody radiation it emits. As the temperature increases, the amount of radiation emitted also increases, and the peak wavelength shifts to shorter wavelengths.

## What is the ultraviolet catastrophe in relation to blackbody radiation?

The ultraviolet catastrophe refers to a theoretical problem in classical physics where the predicted amount of blackbody radiation at short wavelengths is infinite. This problem was resolved with the development of quantum mechanics and Planck's law.

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