Q. The concrete wall of a building is 0.11 m thick. The temperature inside the building is 23.2°C, while the temperature outside is 0.0°C. Heat is conducted throughout the wall. When the building is unheated, inside temperature falls to 0.0°C, and heat conduction ceases. However, the wall does emit radiant energy when its temperature is 0°C. The radiant energy emitted per second per square meter is the same as the heat lost per second per square meter due to conduction. What is the emissivity of wall?