• # question_answer For an opaque plane surface, the irradiation, radiosity and emissive power are respectively 20, 12 and $10\,\,\text{W/}{{\text{m}}^{\text{2}}}.$ what is the emissivity of the surface? A) 0.2                               B) 0.4C) 0.8                               D) 1.0

Radiosity, $J=\varepsilon {{E}_{b}}+(1-\varepsilon )G$ Where G = Irradiation $=20\,\,\text{W/}{{\text{m}}^{\text{2}}}$ ${{E}_{b}}=$ emissive power $=10\,\,\text{W/}{{\text{m}}^{\text{2}}}$ $I=12\,\,\text{W/}{{\text{m}}^{\text{2}}}$ $12=\varepsilon \times 10+(1-\varepsilon )\times 20$ $\varepsilon =0.8$