Online Etymology Dictionary

light (n.): "brightness, radiant energy, that which makes things visible," Old English leht, earlier leoht "light, daylight; spiritual illumination," from Proto-Germanic *leukhtam (source also of Old Saxon lioht, Old Frisian liacht, Middle Dutch lucht, Dutch licht, Old High German lioht, German Licht, Gothic liuhaþ "light"), from PIE *leuk- "light, brightness" (source also of Sanskrit rocate "shines;" Armenian lois "light," lusin "moon;" Greek leukos "bright, shining, white;" Latin lucere "to shine," lux "light," lucidus "clear;" Old Church Slavonic luci "light;" Lithuanian laukas "pale;" Welsh llug "gleam, glimmer;" Old Irish loche "lightning," luchair "brightness;" Hittite lukezi "is bright").

The -gh- was an Anglo-French scribal attempt to render the Germanic hard -h- sound, which has since disappeared from this word. The figurative spiritual sense was in Old English; the sense of "mental illumination" is first recorded mid-15c. Meaning "something used for igniting" is from 1680s. Meaning "a consideration which puts something in a certain view" (as in in light of) is from 1680s. Short for traffic light from 1938. Quaker use is by 1650s; New Light/Old Light in church doctrine also is from 1650s. Meaning "person eminent or conspicuous" is from 1590s. A source of joy or delight has been the light of (someone's) eyes since Old English:
Ðu eart dohtor min, minra eagna leoht [Juliana].
Phrases such as according to (one's) lights "to the best of one's natural or acquired capacities" preserve an older sense attested from 1520s. To figuratively stand in (someone's) light is from late 14c. To see the light "come into the world" is from 1680s; later as "come to full realization" (1812). The rock concert light-show is from 1966. To be out like a light "suddenly or completely unconscious" is from 1934.
light (adj.1): "not heavy, having little actual weight," from Old English leoht "not heavy, light in weight; lightly constructed; easy to do, trifling; quick, agile," also of food, sleep, etc., from Proto-Germanic *lingkhtaz (source also of Old Norse lettr, Swedish lätt, Old Frisian, Middle Dutch licht, German leicht, Gothic leihts), from PIE root *legwh- "not heavy, having little weight" (source also of Latin levis "light;" see lever). The adverb is Old English leohte, from the adjective.

Meaning "frivolous" is from early 13c.; that of "unchaste" from late 14c., both from the notion of "lacking moral gravity" (compare levity). Of literature from 1590s. Light industry (1921) makes use of relatively lightweight materials. The notion in make light of (1520s) is "unimportance." Alternative spelling lite, the darling of advertisers, is first recorded 1962. Light horse "light armed cavalry" is from 1530s. Light-skirts "woman of easy virtue" is attested from 1590s. Lighter-than-air (adj.) is from 1887.
light (v.2): "to shed light; to set on fire," late Old English lihtan (Anglian), liehtan (West Saxon), originally transitive, "to ignite, set on fire," also in a spiritual sense, "to illuminate, fill with brightness." It is common Germanic (cognates: Old Saxon liohtian, Old High German liuhtan, German leuchten, Gothic liuhtjan "to light"), from the source of light (n.).

Meaning "furnish light for" is from c. 1200; sense of "emit light, shed light, shine" is from c. 1300. Buck writes that light is "much more common than kindle even with fire, and only light, not kindle, with candle, lamp, pipe, etc." To light up is from c. 1200 as "give light to" (a room, etc.); 1861 in reference to a pipe, cigar, etc. Related: Lighted; lighting.
light (v.1): "to touch down," as a bird from flight, "get down or descend," as a person from horseback, from Old English lihtan "to alight; to alleviate, make less heavy," from Proto-Germanic *linkhtijan, literally "to make light," from *lingkhtaz "not heavy" (see light (adj.1)). Apparently the etymological sense is "to dismount" (a horse, etc.), and thus relieve it of one's weight."

Alight has become the more usual word. To light on "happen upon, come upon" is from late 15c. To light out "leave hastily, decamp" is 1866, from a nautical meaning "move out, move heavy objects" (1841), a word of unknown origin but perhaps belonging to this word (compare lighter (n.1)).
light (adj.2): "not dark," Old English leoht "luminous, bright, beautiful, shining; having much light," common Germanic (cognates: Old Saxon and Old High German lioht, Old Frisian liacht, German licht "bright"), from the source of Old English leoht (see light (n.)). Meaning "pale-hued" is from 1540s; prefixed to other color adjectives from early 15c.