Etymology
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red-light (adj.)

a red light as a sign to stop is from 1849, long before traffic signals; see red (adj.1) + light (n.). As the name of a children's game (in reference to the traffic light meaning) it is recorded from 1953.

The use of red-light district to indicate a city district with many brothels is by 1896.

On a few blocks east of the Bowery, in what was known as the Red Light district, there are still a few houses of this character. The Red Light district was so called because the hall light in disreputable houses had a red globe or shone through red curtains covering the transom of the hall door. A red light before a cigar store, cider room or coffee room indicated its purpose. The Parisian licensed brothel has a red lantern with the number of the house over the door. [I.L. Nascher, M.D., "The Wretches of Povertyville," Chicago, 1909] 
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light bulb (n.)
also lightbulb, 1884, from light (n.) + bulb (n.). Changing one as figurative of something easy to do is from 1920s; jokes about how many of a certain type it takes to change one date from 1971.
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candle-light (n.)
also candlelight, Old English candelleoht "the light of a candle;" from candle + light (n.). As "time at which candles are lit" 1660s, "an expression much used in places or regions where no correct standard of time is easily accessible" [Century Dictionary, 1895].
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black-light (n.)
"light rays beyond the visible spectrum," 1927, from black (adj.) + light (n.).
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light-headed (adj.)
also lightheaded, 1530s, "dizzy," from light (adj.1) + -headed. Of persons or actions, "frivolous, vain, thoughtless," from 1570s. Related: Light-headedness.
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gas-light (n.)

also gaslight, "light, or a provision for light, produced by combustion of coal gas; a gas-jet," 1808, from (illuminating) gas (n.1) + light (n.). Related: Gas-lighted; gas-lighting; gaslighting

According to Wiktionary, "The verb sense derives from the 1938 stage play Gas Light, in which a husband attempts to convince his wife and others that she is insane by manipulating small elements of their environment."

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light-fingered (adj.)
"thievish, dexterous in taking," 1540s, from light (adj.1) + finger (n.).
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light-hearted (adj.)
also lighthearted, "cheerful," c. 1400, from light (adj.1) + -hearted. Related: Light-heartedly; light-heartedness.
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light-year (n.)
also lightyear, "distance light travels in one year" (about 5.87 trillion miles), 1888, from light (n.) + year.
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arc-light (n.)
"light produced by an electric arc," 1871, from arc (n.) + light (n.). Related: Arc-lamp.
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