Radiation

Black body

• A black body is an ideal emitter
• Energy radiated can be calculated using Stefan-Boltzmann Law

• The radiation emitted by the body, , is referred to as the black body emission and is often denoted by Eb . It is the total energy radiated over all wavelengths.
• Eb,λ , is the monochromatic emissive power.
• At any temperature, Eb,λ has a maximum value at a particular wavelength
• As the temperature increases, the maximum occurs at shorter wavelengths (Wein's Law)
• The visible range extends from about 0.7 microns (red) to 0.4 microns (violet)
• The peak for solar energy lies within this range
• Colour of star depends on temperature of surface (Wein's Law)
• Cold stars red. Hot stars blue (Wein's Law)

Grey body

• As an engineering approximation, surfaces are often assumed to be grey, and the radiation emitted is calculated using:

Transmission, absorption, and reflection

• τ = fraction of radiation transmitted
• α = fraction of radiation absorped
• ρ = fraction of radiation reflected

• Opaque transmit no radiation
• Reflection may be specular as for mirror or diffuse
• tau, alpha, rho are function of wavelenght, but we approximate with constant value to particular temperature range

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