Q: Which “light” came first, the one that refers to illumination or the one that means not very heavy? Is one of them the source of the other?
A: The “light” that shines and the one that’s easy to carry both appeared in Old English, the language of the Anglo-Saxons, but they aren’t etymologically related.
As John Ayto explains in his Dictionary of Word Origins, the “light” that refers to illumination comes from the Indo-European root leuk- (bright, brightness), source of the prehistoric Germanic leukhtam (to produce light) and the Old English léoht (bright).
That Indo-European root also gave Greek leukós (white) and Latin lux (light), Ayto says. Leukós, in turn, is the source of our word “leukemia,” while lux gave us such words as “lucid,” “luster,” “luminous,” and “Lucifer” (literally light-bearer).
The “light” that’s easy to carry comes from the prehistoric Germanic term lingkhtaz, a close relative of the source of the English word “lung,” which etymologically means “something full of air and not heavy,” according to Ayto.
Getting back to your question about which “light” came first, scholars disagree on the dating of some relevant Old English manuscripts, so it’s impossible to give a definite answer.
All we can say is that the adjective meaning bright, the noun for brightness, and the adjective meaning of little weight may all date from the 700s in writing. The verb that means to illuminate came a couple of centuries later.
The earliest example of the noun in the Oxford English Dictionary, spelled leoht in Old English, is from Beowulf, an epic poem that some scholars date at about 725:
“Him of eagum stod ligge gelicost leoht unfæger” (“a dreadful light, more like a flame, shoots forth from his eyes”). The description is of the monster Grendel during a battle with Beowulf.
The first OED example of the adjective meaning “bright, shining, luminous” is from the Vespasian Psalter, an illuminated manuscript that the British Library dates to the second quarter of the 700s:
“Bibod dryhtnes leht inlihtende egan” (“The commandment of the Lord is luminous, enlightening the eyes”). In the interlinear manuscript, the Old English leht is a translation of the Latin lucidum (bright, shining, clear). Many modern versions of Psalm 19:8 use “pure.”
The first Oxford example of the adjective “light” meaning “of little weight, not ponderous” is from a riddle that some scholars believe may have been written in the early 700s, though the earliest manuscript containing the riddle dates from the late 900s.
Here’s the citation, with “lighter” spelled leohtre, from Riddle 40 in the Exeter Book, a collection of Anglo-Saxon poetry at the Exeter Cathedral Library in southwest England:
“Leohtre ic eom micle þonne þes lytla wyrm þe her on flode gæð fotum dryge” (“I am much lighter than this little bug that walks on the water with dry feet”).
The first OED example of the verb “light” meaning to “give or shed light” (that is, to shine) is from the Gospel of John in the West Saxon Gospels, written around 990: “Þæt leoht lyht on ðystrum” (“the light shines on darkness”), from John 1:5-9.
The word “light” has many other meanings, such as understanding (“The study sheds light on a problem”), a notable person (“He’s one of England’s brightest lights”), and a match (“Do you have a light?”), but we’ll save them for another day.