The process of heat transfer from a body by virtue of its temperature with out involvement of intervening medium is called Radiation. The radiant energy is transported by electromagnetic waves because these waves can travel through vacuum.

The Radiation emitted by a body by virtue of its temperature is called Thermal Radiation. It is an inherent property of all bodies.

According to Prevost theory of heat exchanger, every body emits and absorbs radiant energy continuously as long as its temperature is above 0 K.

At low temperature, the emission rate is small while at higher temperatures it increases rapidly as 4th power of absolute temperature.

At ordinary and moderate high temperature, mostly longer waves(infrared) are emitted but at very high temperatures shorter waves are also emitted.

**Properties of Thermal Radiation:**

i) It travels through empty space with the velocity of light.

ii) It undergoes Reflection, Refraction and total internal reflection obeying the same law as light.

iii) It exhibits the phenomenon of interference, diffraction and polarisation.

iv) It exerts a small, but finite pressure on the surface on which it is incident. This is called as pressure of thermal radiation.

v) It obeys inverse square law

Some important terms related to Thermal Radiation are Spectral Energy Density, Total Energy Density, Emmisive Power & absorptive power

**Spectral Energy Density**

Spectral Energy Density for a particular wavelength is the energy per unit volume per unit range of wavelength.

**Total Energy Density**

Total energy density of thermal radiation at any point is the total radiant energy per unit volume around that point due to all wavelengths.

**Emmissive Power**

The emissive power of a body at a given temperature and for a given wavelength, is defined as the ratio radiant energy absorbed for a second by unit surface area of the body per unit wavelength range.

**Absorptive power**

The absorptive power of a body at a given temperature and for a given wavelength is defined as the ratio of radiant energy absorbed per second by unit surface area of the body to the total energy falling per second on the same area.

**Black body and its Radiation**

Energy distribution in black body radiation

Laws of Black body Radiation

Wien's LawRayleigh-Jeans LawPlanck's Law

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