Short answers, pulled from the story.
Light, or visible light, is electromagnetic radiation that the human eye can perceive. It spans wavelengths of roughly 400 to 700 nanometres, corresponding to frequencies of 750 to 420 terahertz, sitting between infrared and ultraviolet.
The speed of light in vacuum is exactly 299,792,458 metres per second, approximately 186,282 miles per second. It is one of the fundamental constants of nature, and all forms of electromagnetic radiation move at this same speed in vacuum.
Light behaves as both a particle and a wave. Isaac Newton argued in 1675 that light was made of corpuscles, while Huygens and Young supported a wave theory, and quantum mechanics later pictured light as in some sense both and in another sense neither.
Ole Rømer, a Danish physicist, conducted an early measurement in 1676 by observing Jupiter and its moon Io through a telescope. Galileo had attempted a measurement in the seventeenth century, and later work by Fizeau, Foucault and Michelson refined the value.
A photon is a single, massless quantum of light and a boson of spin 1. Gilbert N. Lewis named these light quanta photons in 1926, building on Max Planck's 1900 idea of quanta and Einstein's 1905 explanation of the photoelectric effect.
Yes, light exerts physical pressure because photons transfer their momentum when they strike an object. A one-milliwatt laser pointer exerts about 3.3 piconewtons, and at larger scales light pressure can spin asteroids faster and is studied for solar sails.