“Chasing rainbows” means to pursue a “useless quest,” and originated at least as early as 1450.
The idea of the elusive, evasive rainbow, always tantalizingly one step ahead of us but never within reach, is an interesting one. It came from old English literature and is frequently used. In fact, a film made in 1930 was named after the common phrase.
Science has always maintained that finding the end of a rainbow is impossible because it’s an optical illusion which requires a certain amount of distance from the viewer to be seen. However, the camera phone now allows us to snap photos of rainbows apparently touching down in rather mundane places like the middle of a highway.
These images perhaps lend credence to people’s claims to have driven right through a rainbow, often describing it as a semi-mystical experience. Richard Dawkins, in his 1998 book, Unweaving the Rainbow, makes the opposite argument: that an understanding of scientific principles can only increase our appreciation of the wonders of the universe, rather than diminish it.
I’ve read the description, in science texts, of how rainbows appear due to the right conjunction of sunlight and moisture in the air. The knowledge doesn’t take away from my pleasure in seeing a rainbow.