Short answers, pulled from the story.
Visible light spans wavelengths from 400 to 700 nanometres, a narrow band of electromagnetic radiation that human eyes can perceive. This range sits between the longer infrared waves and the shorter ultraviolet rays on the electromagnetic spectrum.
Ole Rømer conducted the first successful measurement of the speed of light in 1676. He observed the motions of Jupiter and its moon Io to calculate that light takes about 22 minutes to traverse the diameter of Earth's orbit.
James Clerk Maxwell formulated the electromagnetic theory of light and published his equations in 1873. Heinrich Hertz experimentally confirmed these equations by generating and detecting radio waves that behaved exactly like visible light.
The human eye detects visible light when the visual molecule retinal undergoes a conformational change upon being struck by photons. This process triggers the sensation of vision and occurs within the three types of cone cells that respond differently across the visible spectrum.
The speed of light in a vacuum is exactly 299,792,458 metres per second. Albert A. Michelson refined the measurement of this speed in 1926 by timing light's round trip from Mount Wilson to Mount San Antonio in California.