Our transcription: Like sound waves in the air, seismic waves move in three dimensions forming spherical wave fronts as they pass through our planet.
Where the seismic waves encounter rock layers of differing densities, the waves reflect off the boundaries between the layers in much the same way light reflects off a mirror.
These waves are recorded on seismographs during earthquakes allowing geophysicists to see how long they take to travel down to the boundary, reflects off it, and return to the surface.
From the amount of time needed for the round trip, scientists can calculate the depth of the boundary, and thus learn more about the deep structure of the Earth.
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