What do you mean by the term "energy density of radiation "

[Sampad Saha]

What do you mean by the term "energy density of radiation " between the frequency range f and f +df ?

 

[jtbell]

Can you tell us the context or equation where you saw this? There are different kinds of "densities."

As a wild-assed guess, since you mentioned a frequency range, you are dealing with energy as a function of frequency (e.g. a graph of energy versus frequency), and the "density" means the amount of energy per unit of frequency. But this is only a guess!