How It Works: Why Seismic Waves Refract Inside Earth

⚙️ How It Works: Why Seismic Waves Refract Inside Earth

Just like light refracts when passing from one medium to another, seismic waves refract when they pass through layers of different density inside Earth. As depth increases, pressure and temperature change, which changes the density and rigidity of rock — and therefore the wave speed.

Seismic waves generally speed up as they go deeper into the mantle (rock becomes denser and more rigid under pressure). Because speed increases continuously with depth, wave paths curve gradually — like light in a medium with a gradually changing refractive index. This curved path means seismic waves arrive at seismograph stations at unexpected positions, allowing scientists to deduce the density profile of Earth's interior.

When waves cross a sharp boundary (like the mantle-core boundary), there is a sudden change in speed and direction — like light hitting a glass surface — creating the distinct shadow zones observed.