In a rainbow, the shortest visible wavelength of light is approximately 400nm (blue) and the longest 700nm (red), where all others colors outside this range are invisible to humans.
Except not quite. Artal et. al. demonstrate that the eye’s visual acuity for infrared light (1000nm), is almost the same as for visible green light. The exploited effect in the eye converts two-invisible photons into a single visible one. Importantly, this demonstrated sensitivity to infrared light could enable future ophthalmic devices to help patients with eye conditions, such as cataracts, that make them opaque to visible light. Continue reading It takes two to see (infrared photons anyway)