Letters from Aldwin

There is a band of indigo, 419 to 446 nanometers, that used to be in every day a child spent outside. It is still in sunlight. It is almost absent from the white LEDs we read by. A new study says that matters.

Researchers at Cincinnati Children's and UAB gave tree shrews tiny goggles that push one eye toward nearsightedness, then lit them with different lights. Indigo completely prevented the myopia.

The strange part is the road there. In mice the protective color was violet, near 380 nanometers, sensed by a protein called OPN5. But tree shrews and humans share a lens that blocks almost everything below 400, so the team hunted for the next color the lens would let through. Indigo does the job: it reaches the retina, and the eye reads it as a signal not to stretch too long.

The eye does more than see. It checks the light for a color that means “outside,” and when the color is missing, the eye grows wrong. The lamps we invented left out a shade the eye was still waiting for.

Nobody is putting indigo lamps in classrooms yet. But it is a quiet reminder of how we shape growing things without meaning to, and how small the fix can be: a color given back.