Heat and Mass Transfer
When α is absorptivity, ρ is reflectivity and τ is transmissivity, then for a diathermanous body,

α + ρ = 1 and τ = 0
α = 0, ρ = 0 and τ = 1
α = 1, ρ = 0 and τ = 0
α = 0, ρ = 1 and τ = 0

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Heat and Mass Transfer
Fourier's law of heat conduction is (where Q = Amount of heat flow through the body in unit time, A = Surface area of heat flow, taken at right angles to the direction of heat flow, dT = Temperature difference on the two faces of the body, dx = Thickness of the body, through which the heat flows, taken along the direction of heat flow, and k = Thermal conductivity of the body)

k. (dT/dx)
k. A. (dT/dx)
k. A. (dx/dT)
k. (dx/dT)

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