What is radiation?
The movement of electromagnetic wave without any physical contact of a body is called radiation. Example – light coming from the sun. Heat transfer form hoter to colder body.
All bodies radiate some energy except at zero Calvin. The energy emitted by the body is in the form of packets or quanta of energy called photon.
When the radiation is incident on the surface of a body some fractional part of energy is being absorbed by the surface, some fractional part of energy being transmitted by the surface and some fractional part being reflected by the surface.
The fractional part which is absorbed by the body is called absorbitivity of that body and it is denoted by sigma. The fractional part which is reflected by the body is called reflectivity and it is denoted by row. The fractional part which is transmitted by the body is called transmissivity and it is denoted by tow. Mathematical expressions for this
ρ + α + τ = 1
Where ρ is reflectivity, α is absorbitivity and τ is transmissivity.
The value of these are depend upon
1 nature of surface of the body
2 Temperature of the body. – greater the temperature greater will be the radiation
3 wavelength of the incident rays.
The body that absorbed all the radiation, and does not trasmite or reflect any radiation is called black body. i. e α =1 and ρ =τ=0. It doesn’t mean that for the black body the body should be black. For example snow which is white in colour and whose absorbitivity is 0.955 that is very near to the 1, hence it is a black body.
The body which which reflects all the radiation is called white body. i.e α = τ = 0, ρ = 1.
It is a hypothetical body because the value of absorbitivity, reflectivity and transmissivity does not varies with temperature. ρ + α + τ =1