Thermal Radiation

What is Thermal Radiation?

Thermal radiation is one of the main ways of transferring thermal energy, alongside convection and conduction.
Thermal radiation is the transfer of energy by electromagnetic waves.
Unlike conduction and convection, it does not need particles to travel and can transfer energy through a vacuum.

Certain surfaces absorb and emit thermal radiation much more effectively.
- That's why a black car heats up much quicker than a white car on a hot day.
Any surface which is a good absorber of radiation will also be a good emitter.
- Black surfaces emit and absorb radiation well while white surfaces are poor at absorbing and emitting.

A Black body is defined as a body that emits all the radiation incident upon it in the same pattern of wavelengths.

Solids can absorb many more wavelengths than atmospheric gases.
Depending on the color of a solid, it is possible to determine what wavelengths it cannot absorb.
This is because the wavelengths that aren't absorbed are reflected, and we can see them!
- For example, a green object reflects green light, which makes it appear green to us.
- It absorbs all non-green wavelenghts.
A black object absorbs all wavelengths.
Likewise, a white object doesn't absorb any wavelengths and reflects them all back instead.

Luminosity is defined as the total amount of radiation a black body emits per second.
In essence, luminosity defines power, as it is also calculated as energy over time.

As a black body emits as much power as it absorbs, the Stefan-Boltzmann law works for both emission and absorption problems involving black bodies.

Emissivity is defined as the ratio of energy radiated by a body compared to the theoretical energy a perfect black body at the same temperature would emit.
Thus by definition the emissivity of a perfect black body is 1.

Imagine a yellow light bulb giving out light.
It emits some radiation at some wavelengths, but the most intense light is at the yellow wavelengths.
If graphed, we can see how this looks like.

Maximum intensity occurs at a specific wavelength.
The graph rises steeply until the maximum point, but drops off slowly after the peak, causing an asymmetrical graph.
The intensity approaches the x-axis as an asymptote.

From the graph we can also see that the emitted wavelength is also dependent on temperature.
Lower temperatures have larger wavelengths for their peaks, but the intensity at the peak is smaller.
- This is why blue fire is hotter than red fire!

Wien's law states that as temperature goes up, the peak wavelength goes down, showing that they have an inversely proportional relationship.
This also means that:
- peak wavelength * temperature = constant

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