# Rayleigh-Jeans Law difference between the frequency and wavelength forms [duplicate]

I don't understand the conversion between the frequency and wavelength forms of the Rayleigh-Jeans equation for spectral radiance. According to the Wikipedia article on the Rayleigh-Jeans law:

However, if λ=c/f and we substitute c/f for lambda in the left side of the equation it does not come out to be the same as the right side of the equation. In other words:

Which seems to be different than the right side of the first equation above.

• Have you read the section in the very Wikipedia article you link? Commented Jan 2, 2015 at 15:38
• This is a very common query, probably a duplicate exists. The solution is that $B_{\lambda}\ d\lambda = B_{\nu}\ d\nu$, so $B_{\nu} = B_{\lambda}\ d\lambda/d\nu$. Commented Jan 2, 2015 at 15:54
• @RobJeffries That is Planck's Law, not Rayleigh-Jeans Law. Commented Jan 2, 2015 at 16:06
• The Rayleigh-Jeans law is just the low frequency approximation of the Planck distribution! The solution to your problem is as I wrote above or as given in the duplicate. Commented Jan 2, 2015 at 16:08
• Commented Jan 2, 2015 at 16:10