# Laws Of Radiation Heat Transfer – Wien’s Law, Kirchhoff’s Law And Stefan-Boltzmann Law

### Wien’s displacement law and equation

Wien’s law states that in a black body radiation the wavelength of radiation corresponding to the maximum energy is inversely proportional to the temperature of the body.

Let λ

_{m}=Wavelength at maximum energyT = Temperature of body.

Then according to Wien’s law

The Constant is known as Wien's displacement constant, and it is equal to 2.897771955….×10

^{−3}m KAccording to the Wien’s law, at lower temperature, higher the wavelength of maximum energy radiation, and at a higher temperature the lower the wavelength of thermal radiation. That is why in visible spectrum of radiation, the very hot object emits bluer light than the cool object.

### Kirchhoff’s law of thermal radiation

Kirchhoff’s law states that the emissivity of the body at a particular temperature is numerically same as the absorptivity of the body at the same temperature.

absorptivity α = emissivity ɛ

### Stefan-Boltzmann law of radiation

Stefan-Boltzmann law states that the emissive power of the black body is directly proportional to the fourth power of its absolute temperature.

σ = Constant of proportionality known as Stefan–Boltzmann constant

### Equation for heat transfer by radiation

Consider two bodies at temperature T

_{1}and T_{2}Where T

_{1}>T_{2}Then Heat transfer through radiation will be Q

Q = Q

_{1}-Q_{2}According to Stefan-Boltzmann law

Where σ = Stefan-Boltzmann constant = 5.67 x 10

^{-8}wm^{-2}K^{-4}F = A factor depending on geometry and surface properties

F =1 in the case of black body entirely enclosed by other body.

F = emissivity (ɛ), for non-black body enclosed by other

A = Area of the body in m

^{2}T

_{1}and T_{2}= Temperature in kelvin, K