Particles collide with molecules in atmosphere and cause them to emit light. Travel through solid and liquid. Travel through Solids only. Detect seismic waves with seismographs. Calculate the velocity of seismic waves by measuring travel times and distances.
Seismic waves travel faster in denser rock.
S-waves can't pass through the liquid core. Shadow zone . Sharp jumps in seismic velocity are called discontinuities . Mohorovicic Discontinuity separates crust and mantle. Origin of layering and discontinuities Several models.