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Radiation waves may travel in unusual patterns compared to conduction heat flow. Radiation allows waves to travel from a heated body through a cold non-absorbing or partially absorbing medium and reach a warmer body again. [14] An example is the case of the radiation waves that travel from the Sun to the Earth.
The effects of electromagnetic radiation upon living cells, including those in humans, depends upon the radiation's power and frequency. For low-frequency radiation (radio waves to near ultraviolet) the best-understood effects are those due to radiation power alone, acting through heating when radiation is absorbed.
The Stefan–Boltzmann law gives the power emitted per unit area of the emitting body, = (,) Note that the cosine appears because black bodies are Lambertian (i.e. they obey Lambert's cosine law ), meaning that the intensity observed along the sphere will be the actual intensity times the cosine of the zenith angle.
The heating effect varies with the power and the frequency of the electromagnetic energy, as well as the inverse square of distance to the source. The eyes and testes are particularly susceptible to radio frequency heating due to the paucity of blood flow in these areas that could otherwise dissipate the heat buildup. [7]
Thus the ratio E(T, i) / a(T, i) of emitting power to absorption ratio is a dimensioned quantity, with the dimensions of emitting power, because a(T, i) is dimensionless. Also here the wavelength-specific emitting power of the body at temperature T is denoted by E(λ, T, i) and the wavelength-specific absorption ratio by a(λ, T, i).
Prior to Kirchhoff's studies, it was known that for total heat radiation, the ratio of emissive power to absorptive ratio was the same for all bodies emitting and absorbing thermal radiation in thermodynamic equilibrium. This means that a good absorber is a good emitter. Naturally, a good reflector is a poor absorber.
It might seem like a simple question. But the science behind a blue sky isn't that easy. For starters, it involves something called the Rayleigh effect, or Rayleigh scattering. But that same ...
is the outgoing heat transfer from the area d ω {\displaystyle d\omega } is the solid angle subtended by the infinitesimal 'target' (or 'aperture') area d A a {\displaystyle dA_{a}} θ {\displaystyle \theta } is the angle between the source area normal vector and the line-of-sight between the source and the target areas.