UPSC Combined Geo-Scientist, Mains · 2016 · Mains Paper II

Q1(c). Entropy and diffraction

Question

Justify: (i) the net entropy of the Universe increases; (ii) X-rays are used for diffraction by crystals.

An irreversible spontaneous process increases total entropy; X-ray wavelengths are comparable to crystal interplanar spacings.

For the system and surroundings together, the second law gives ΔSᵤₙᵢᵥ=ΔS_(sys)+ΔSₛᵤᵣᵣ≥0.
Crystal spacings are typically of order 10⁻¹⁰ m, comparable to X-ray wavelengths. Scattering from successive planes therefore gives measurable interference angles.
Constructive interference satisfies Bragg' s law, nλ=2dsin θ. Visible wavelengths are too large for ordinary atomic-plane diffraction.

The full model answer

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CoversEntropydiffractionThermodynamics; diffraction