At what point are water droplets too small to make a rainbow?
Some time ago I posted this answer about how rainbows are formed, and the Wikipedia link Trond Hansen posted mentions droplet size relative to the wavelength of light.

For a rainbow to form, the droplet size has to be large enough, relative to the color with the longest wavelength of visible light, for it to be refracted before reflecting off the backside of the droplet. Refraction occurs because, as the wavefront encounters the change in density between air and water, it slows down. This implies that a droplet would have to be greater than half the wavelength of red light, which is about 740 nm, or about 370 nm.1 Also, the light responsible for producing the rainbow enters the droplet off-center and reflects off of the backside of the droplet off-center, so the droplet's diameter would need to be a bit larger than 740 nm.




https://en.wikipedia.org/wiki/Light