This switch between rods and cones takes time, which is what causes the momentary blindness. Of course, the story is not as simple as that – other changes also need to take place. By contrast, the pupil of the eye opens wider in dark conditions, in order to let the maximum of light in, but contracts when there is too much light. All in all, vision is the product of an intricate and coordinated biological system that scientists are still working to understand.