Suppose we have a blackbody at temperature T. Then if we write Planck's law for wavelength, and find the wavelength corresponding to the peak, we get a certain value lambda_max. If, on the other hand, you wrote Planck's law for frequency, and we found the frequency corresponding to peak of that, we get a certain value nu_max. But if you multiply lambda_max and nu_max, you do not get the speed of light. In other words, the peak of Planck's law for wavelength does not correspond to the peak of Planck's law for frequency. How is this possible? Any help would be greatly appreciated. Thank You in Advance.