Please have a look at the Bragg law:

lambda=2*d*sin(theta)

This equation has to be valid for both wave lengths (Cu and Fe -K-alpha).

So you have:

lambdaCu = 2*d*sin(thetaCu) and lambdaFe = 2*d*sin(thetaFe)

Taking the ratio you will have:

lambdaCu/lambdaFe = sin(thetaCu)/sin(thetaFe)

Solving for thetaFe you have to go via:

sin(thetaFe) = sin(thetaCu)*lambdaFe/lambdaCu

That's it.....

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