Radiation

Concepts

If we want something to absorb thermal radiation, what properties are necessary?
If we want something to shed thermal radiation?
What if we want to use sunlight to warm something?
What if we want light but not heat?

What does the spectrum of radiation look like for the sun?
- Which part of this is thermal?
What does the spectrum of radiation look like for the earth?
- Which part of this is thermal?

This law relates the intensity of power at each wavelength with the temperature of an object (ideal black-body).

$B_\lambda(\lambda, T) =\frac{2 hc^2}{\lambda^5}\frac{1}{ e^{\frac{hc}{\lambda k_\mathrm{B}T}} - 1}$

This law relates the total power emitted to the temperature, surface area, and emissivity of an object.

$P = A \epsilon \sigma T^4$

$A$

surface area

square meters

$\epsilon$

emissivity

dimensionless

$\sigma$

Stefan-Boltzmann constant

$5.67\cdot 10^{-8}$ W per square meter per K $^4$

$T$

temperature

Kelvin

The emissivity is how much radiant power is emitted by a substance relative to an ideal black-body emitter.

If we have two objects with the same surface area and temperature, the object with the higher emissivity emits more radiation.

Kirchoff's Law of thermal radiation states that a body in thermodynamic equilibrium, the absorptivity and emissivity are equal.

$P = \epsilon \sigma A (T_1^4 - T_2^4)$

Last updated 4 years ago