An FCC crystal of lattice parameter 0.3615 nm reflects monochromatic X-rays from (111) planes at Bragg angle θ=21.68°. Find the wavelength in nm (three decimal places).
The spacing is $d_{111}=a/\sqrt3=0.20871$ nm. First-order Bragg reflection satisfies λ=2d sinθ. Thus λ=2(0.20871)sin21.68°≈0.1542 nm, which rounds to 0.154 nm. θ is the Bragg angle, not twice that angle.
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