The Shadow Zone

Plate 92 speaks for itself: “Earth's centre was found in the waves that weren't supposed to arrive.”

The Physics

Seismic waves bend at the boundary of Earth's liquid outer core, leaving a shadow zone. Lehmann noticed faint waves inside it in 1936 and deduced a solid inner core.

n₁ sin θ₁ = n₂ sin θ₂

• n₁, n₂: slowness (1/wave speed) in each layer, s m⁻¹
• θ₁, θ₂: angles to the boundary normal, rad

The liquid outer core leaves a shadow ≈ 104°–140°. Inner core ≈ 1220 km; outer core ≈ 3480 km.

Sources

• I. Lehmann, Publ. Bur. Cent. Séismol. Int. A14, 87 (1936).
• A. M. Dziewonski, D. L. Anderson, PREM, Phys. Earth Planet. Inter. 25, 297 (1981).

On the poster

Plate 092, “The Shadow Zone”: the pixel-art plate with its slogan “Earth's centre was found in the waves that weren't supposed to arrive.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.