Hi!

I would like to know how I calculate the ratio between the energy of EM-radiation emitted in one wavelength interval and the energy of EM-radiation emitted in another wavelength interval, for a body with known temperature.

I guess it involves Planck's law, but I'd be grateful if you could explain to me how.

Will it be something like:

, where w is the wavelength, and a,b,c and d are the llimits for the wavelength range?

The only problem is I don't know how to calculate I' (I guess it means the derivative of I).