Bragg's Law: Salt Planes

Plate 56 speaks for itself: “Read the angles.”

The Physics

X-rays reflect strongly from crystal planes when waves from neighbouring planes arrive in step. Measuring the angles of those reflections from rock salt gave the first crystal structure.

n λ = 2 d sin θ

• n: order, integer
• λ: X-ray wavelength, m
• d: plane spacing, m
• θ: glancing angle, °

Rock salt is cubic, a = 564.02 pm. (200) is the strongest reflection.

Sources

• W. L. Bragg, Proc. Camb. Phil. Soc. 17, 43 (1913).
• W. H. Bragg, W. L. Bragg, Proc. R. Soc. A 88, 428 (1913).

On the poster

Plate 056, “Bragg's Law: Salt Planes”: the pixel-art plate with its slogan “Read the angles.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.