q calculate the longest wavelength that can be

Question

Q Calculate the longest wavelength that can be analysed by a rock salt crystal of spacing d = 2.82 Ao in the first order.

Data: d = 2.82 Å = 2.82 × 10-10 m; n = 1 ; λmax = ?

Solution:         For longest wavelength, (sin θ) max = 1

∴ 2d (sin θ) max = ^λmax

(Or) ^λmax = 2 ×2.82 × 10^-10 ×1 / 1

^λmax = 5.64 × 10^-10 m