Radiation is everywhere we go! Now when you hear radiation, you may be thinking of "nuclear" radiation; this type comes from particles such as protons, alphas (Helium nuclei), electrons, and neutrons as well as gamma rays. Here, we will be talking about larger wavelength electromagnetic waves (EM) such as in the infrared spectrum, or as it is commonly referred to as, thermal radiation. What is fascinating about this topic is you and I give off thermal radiation!
We discussed a little on radiation at the beginning of this guide, but let's do some recap. Radiation involves radiation emission (E) from a surface and may also involve the absorption of radiation from surrounding irradiation (G). If the surface is surrounded by fluids, convection can also occur (we will work through plenty of this in later tutorials and lessons).
Let's first look at energy outflow due to emission.
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$$ E = \epsilon E = \epsilon \sigma T_s^4 $$
If a surface has a temperature above absolute zero (0K), it will emit thermal radiation, and we can use Eq. (1) to calculate how much energy it gives off! What is crazy is how we can do this with one simple equation that has two inputs, the emissivity $ \epsilon $ and the surface temperature $ T_s $. The emissivity is between 0 and 1 and dictates how readily the surface is to give off energy compared to a blackbody at the same temperature. The last piece to Eq. (1) is the Stefan-Boltzmann constant which has a value of $ \sigma = 5.67E-8 \frac{W}{m^2 • K^4} $.
Notice in the last paragraph, I mentioned a blackbody. A blackbody is a surface or material that absorbs all incoming radiation and only emits thermal radiation (hence, it has an $ \epsilon = 1 $). For blackbodies, since their $ \epsilon = 1 $, their emissive power can be written as
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$$ E_b = \sigma T_s^4 $$
Now let's take a look at absorption due to irradiation. If an a surface is irradiated by energy flux G (units $ \frac{W}{m^2•K^4} $), there will be energy absorbed by the surface for non-zero $ \alpha $, given
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$$ G_{abs} = \alpha G $$
Here, $ \alpha $ is the absorptivity of a given material and is between 0 and 1.
Let's slightly shift focus and look at net radiation heat flux. One simplification we can make is for a small gray surface (where $ \alpha = \epsilon $) with large surroundings, the net radiation heat flux is
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$$ q_{rad}^" = \epsilon E_b(T_s) - \alpha G = \epsilon \sigma (T_s^4 - T_{surr}^4) $$
For close surface temperature ($ T_s $) and surrounding temperature ($ T_{surr} $), the following can be used for $ q_{rad}^" $.
$$ q_{rad}^" = h_r (T_s - T_{surr}) $$
$$ h_r = \epsilon \sigma (T_s + T_{surr}) (T_s^2 + T_{surr}^2) $$
We can also combine our $ q_{rad}^" $ with convection, but that will be for a later tutorial.
The origin of thermal radiation is from emission from matter at an absolute temperature ($ T > 0 $). Emission is due to oscillations and transitions of electrons in the matter. When an electron is contacted with energy (could be from heat), it excites and goes up to higher energy states. It is when electrons drop to lower energy states that energy is released, oftentimes in the form of EM waves.
This thermal energy that is released can contact other surfaces, heating them up, or it could be reflected back onto itself from another surface. Emission from a gas or semitransparent solid or liquid is a volumetric phenomenon whereas emission from a opaque solid or liquid is a surface phenomena. Emission typically originates from atoms and molecules about $ 1 \mu m $ from the surface. We will focus a lot on surface radiation in this guide!
One final thought, we will be viewing radiation in this guide as particles called photons or quanta. Radiation has a wavelength ($ \lambda $) and a frequency ($ \nu $). Together, they form an equation that relates to their propagation speed, the speed of light, where $ c = 2.998E8 \frac{m}{s} $ in a vacuum.
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$$ \lambda = \frac{c}{\nu} $$
That is all I want to touch on for radiation properties, but there is much more to dive into next!