Whose bright idea was it to use the same word for 'visible electromagnetic radiation' and 'low in weight'?
The OED tells me they both descended from the Old Enligsh word léoht, which has the meanings:
1. adj light, not heavy; slight, easy, trifling, inconsiderable; quick, agile; gentle; 2. n (-es/-) light, daylight; power of vision; luminary; world; 3. adj luminous, bright, light, clear, resplendent, renowned, beautiful.
Perhaps because light can be seen, but not touched, felt (as heat) but not weighed. It surrounds us, and yet seems to be nothing. My guess would be that the use as an adjective is a (very) dead metaphor. All the adj meanings seem traits which could apply to light, and therefor ones which one might invoke by metaphorically calling something "light". Over time frequent used of a metaphorical term like that "kills" the metaphor so that while we understand its meaning, we no longer feel the tension as it pulls against its original meaning.