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"In geometrical optics, shadows cast by obstacles would extend indefinitely -- if you're in the shadow, there's no light; outside of it, you see light. But wave optics introduces a different behavior -- waves bend around obstacles and interfere with each other, creating a sequence of bright and dim fringes due to constructive and destructive interference."

Lessons in geometrical optics would emphasize applications to battlefield scopes; lessons in acoustics would drop examples from music in favour of depth sounding and sound ranging.

Huygens explained how wave theory accounts for geometrical optics.

Geometrical optics describes light propagation in terms of “rays”.

Considerable advance had been made in the subject of geometrical optics; the rectilinear propagation of light and the laws of reflection having been known to the Greeks and Arabians, whilst Willebrod Snellius, Professor of Mathematics at Leyden, had correctly enunciated the laws of refraction very early in the seventeenth century.