Hi All, I'm trying to understand the radiation mode of heat transfer. I have been reading the book by Incropera - "Fundamentals of heat and mass transfer" and have a few very basic questions. 1) Firstly, thermal radiation is the energy emitted by matter that is at non-zero temperature. Radiation may be emitted from solid surface or liquids or gases. The energy of radiation is transported by electromagnetic waves (or alternatively, photons). While the transfer of energy by conduction or convection requires presence of material medium, radiation does not. In fact, radiation occurs most efficiently in a vacuum. QUESTION 1: Does it mean that radiations are emitted by matter (solid or liquid or gases) but transported in a vacuum. I do not get quite correctly when we say that transport through radiation does not involve a material medium like in case of conduction and convection. 2) Definition of black body. Be definition, the upper limit of the energy released by the surface is given by Stefan Boltzman law, which is; Eb= σ * T4 It is also said (in the text that I have been reading) that such a surface is called an ideal radiator or black body. My question is very basic / more of a stupid layman question now. QUESTION 2: It is always said that during cold season, you wear a black shirt, you would not feel cold. I'm trying to relate the above statement to the definition of black body (the ideal radiator in Stefen Boltzman law), by the definition of ideal radiator (the black body), shouldn't the black body (my black shirt-say) emit MORE heat to surroundings than a white shirt?