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## Homework Statement

HI, I am practicing some exam questions to get ready for a repeat, below is the question I am having trouble with.

"The lattice constants of KCl and KBr are 6.29 angstroms, and 6.59 angstroms, respectively.

What would be the first two observed Bragg diffraction angles in each case , if the wavelength of X-rays was 1.932 angstroms? (assume Debye-Scherrer method is used.)

## Homework Equations

Here, Braggs law is relevant,

nλ= 2dsinθ.

## The Attempt at a Solution

Braggs law can be rearranged to give θ = sin^-1(nλ/2dhkl).

the Questions asks for the first two angles for each case, so n=1 and n=2 for the first and second angle.

my two problems, are, how can I use α, the lattice constant to get dhkl?

Since the two Crystals are both FCC, the lattice parameters are a=b=c, and α=β=γ,

dhkl =[1/a^2 + ( h^2 +k^2 + l^2)]^-1/2

I haven't been given any miller indices for h,k &l, so do I assume some, or just use α as dhkl?

Also, will the angles be in units of radians or degrees?

Any help or push in the right direction will be much appreciated.

Cheers