Radiation is the process where energy is transferred by means of electromagnetic waves. For example energy from the sun is brought to the earth by huge amounts of visible waves as well as infrared, and ultra violet waves. This energy transfer do not require any medium.

All bodies regardless of the temperature continuously radiate energy in the form of electromagnetic waves. For example. you may not think that chuck of ice at -10°C do not radiate energy because it is cold. It does radiate energy but not enough to radiate and produce visible light. Even humans radiate energy and again not enough to radiate visible light for anyone to see.

Note: A material that is a good absorber of energy like an object that is totally black and dull is also a good emitter of energy. An object that is a poor absorber of energy like the white polished and shiny object is also a poor emitter of emitter of energy.

The amount of radiant heat that flows in some time t is given by

where e = the emissivity constant. This value is the decimal percentage of how much energy is absorbed. Dimensionless.

s = Stefan Boltzman constant and has a value of 5.67 x 10-8J/s.m2.K4

  T = the temperature of the body in Kelvin
  A = surface area of the body in m2
  t = the time in s

Example 1

Assume that the earth has an average surface temperature of 22°C and is perfect black body . What is the radiant heat of energy per second per square meter that the earth radiates into space.

Example 2

By what factor should the Kelvin temperature of an object be increased to double the radiant energy per second emitted by the object?