Radiation is that mode of heat transfer which does not require any medium for its propagation. The heat energy which is transferred from hotter to colder object by the process of radiation is known as radiant heat or thermal radiation. Radiation is the fastest mode of transfer of heat. Radiant heat is electromagnetic in nature. All bodies emit heat radiations unless the temperature of body is zero Kelvin. When radiant energy is incident on a surface, reflection, absorption and transmission occurs. Black bodies are good absorbers of radiant heat. If the radiant heat falls on the blackened bulb thermometer, it records a rapid rise in temperature. Thus, a blackened bulb thermometer is used for detecting radiant heat. Heat radiations are also detected by differential air thermoscope and by thermopile. When thermal radiation falls on the surface of an object, some part of the energy is absorbed and the remaining part is reflected back. The absorbed radiation increases the object’s temperature. Absorption depends on the nature and colour of the surface. The surfaces which absorb less heat and reflect more heat are called bad absorbers or good reflectors. The ratio between the heat reflected by a surface of the body to the quantity of heat which falls on it in one second is called reflective power of the body and the ratio between the quantity of heat absorbed by the surface of a body to the quantity of heat falling on it in one second is called the absorbing power of the body. Different surfaces have different absorbing and radiating powers. There are various experiments that demonstrate the absorbing and radiating powers of different surfaces. There are many applications of heat radiations in our daily life. Thermo flask or vacuum flask is one of them, which is used to keep hot liquid hot and cold liquid cold for a long time.
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