AA 301 Astrophysical Processes Thermal Radiation and Blackbody Course coordinator Dr. Suman Majumdar

Blackbody Radiation Radiation which is in thermal equilibrium with itself is called Blackbody Radiation.

Blackbody Radiation Radiation which is in thermal equilibrium with itself is called Blackbody Radiation. One can derive different properties of the blackbody radiation from simple thermodynamical arguments.

Blackbody Radiation Radiation which is in thermal equilibrium with itself is called Blackbody Radiation. One can derive different properties of the blackbody radiation from simple thermodynamical arguments.

Blackbody Radiation Radiation which is in thermal equilibrium with itself is called Blackbody Radiation.

Blackbody Radiation Kirchoff’s Law

Distinction between Blackbody and Thermal Radiation In case of blackbody radiation Iν = Bν And for thermal radiation Sν = Bν. Even if all matter is radiating thermally (i.e. Sν = Bν) one is not guaranteed that Iν = Bν unless the medium is optically thick. One can test this by putting τ ≫ 1 in the following solution of the radiative transfer equations

Planck Function

Planck Function

Planck Function Rayleigh-Jeans Limit:

Planck Function Rayleigh-Jeans Limit: Fun fact: These results were found empirically by Rayleigh in 1900 and published by Rayleigh and Jeans in 1905 (after Planck!).

Planck Function Wien Limit:

Planck Function Wien Limit: Fun fact: These results were found by obtained by Wein using classical thermodynamics arguments in 1896 before the publication of Planck results and was proven to work well in the UV but failed in IR.

Different Temperatures

Brightness temperature of 21-cm radiation

Brightness temperature of 21-cm radiation from the Epoch of Reionization Majumdar et al. 2016

Different Temperatures

Different Temperatures