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Question:

For Bragg's diffraction by a crystal to occur, then which of the following is correct for the X-ray of wavelength λ and interatomic distance d?

λ equals 2d

λ is greater than 2d

λ is smaller than 2d

λ is smaller than or equal to 2d

Solution:

The condition for Bragg's diffraction is
2d sin θ = nλ ⇒ sin θ = nλ/2d ≤ 1 ⇒ λ ≤ 2d/n
where n is a positive integer
Hence the maximum value of λ occurs for n=1.
Hence λ ≤ 2d