Seneca the Younger, On Rainbows

"Let us inquire how it is produced. Some authorities say there are certain drops of water that transmit light, while some are too compact to be translucent. Thus the brightness is the effect of the former; the shadow, of the latter; by the intermingling of the two is formed the rainbow, part of which is bright, to wit, that which admits sunlight, part darker, namely, that which has shut out the light and cast a shadow from itself over the objects nearest it Others again deny that this is so. Shade and light they say, might be the cause if the rainbow had only two colours, and thus was made up of light and shade.

But now, though there a gleam of thousand diverse hues,Their changes withal elude the eyes that behold.The hues that touch seem actually one, yet the edges are quite different

[...] Aristotle agrees with this opinion. His words are: Beams of light are reflected by sight from every smooth surface. Now, nothing is smoother than water and air. Therefore, our sight is reflected back on us from thick air. Indeed, where the vision is dull and feeble, the slightest stroke of air checks it."