Light Slows Down When Traveling Through Water or Glass

A flaw of about two micrometers left early images blurry until astronauts installed corrective optics in 1993.

The first wearable lenses appeared around 1286, long before the invention of the microscope or telescope.

In the early 11th century he argued that vision works by light entering the eye and used experiments to show it.

Where the optic nerve leaves the retina there are no light-detecting cells, but the brain fills in the gap.