Optics:

Until a few hundred years before, the only instrument obtainable for observation of visible phenomenon was the human eye. This changed as experimenters prepared microscopes, telescopes, and other devices.

Behavior of Light:

The visible light always takes the shortest path among two points, and it always moves at similar speed. These two rules hold as long as the light remains in a vacuum. Though, if the medium via which light passes is appreciably diverse from a vacuum, and particularly if the medium changes as the light ray travels, such axioms do not exert. When a ray of light passes from air into glass or from glass into air, for illustration, the path of the ray is bent. The light ray also changes direction whenever reflected from a mirror.