How do human eyes adapt to the dark

As soon as the lights go out you find it hard to see anything, but after a while you start seeing things better. Why is this?

Thanks.

pupil reflex accounts only for part of it. You have rods and cones in your retina (look terms up in an encyclopedia if you are unfamiliar with any). We use the rods for night vision and cones for daylight vision. There are three different types of cones, therefore we have color vision in daylight. There is only one type of rod, so at night we don't have color vision ("at night all cats are gray..."). If we switch from light to dark vision, then the rods have to recover from "excessive" light exposure in order to become functional. The rods don't work well with red light, therefore some people use red flashlights to illuminate something in the dark, since this doesn't really stimulate the rods (just our cones), so night vision isn't destroyed if you use a brighter light as long as it's red.

This is the delay in pupil reflex. It is caused by the contraction of the circular and radial muscles around the eye.

In dim light, the circular muscle shrinks (relaxes) and the radial muscle also shrinks, except it does this by contracting. This pulls the iris open and increases the aperture of the lens (amount of light that can get through).

You want more light in darker situations to reach the fovea (yellow spot) and be processed as an image in your brain, hence the aperture is small in the daytime and large in the night.

The Pupil dilates(get bigger) in the dark so more light is able to enter the eye, on a sunny sunny day the little black spot in your eye is the pupil, often a flashlight in your eye on a dark night is somewhat blinding because the pupil is allowing in too much light,

When the light goes off, as a reflex action, our eyes expand so that more amount of light can be taken in..
The opposite happens when we'r suddenly confronted with bright light; our eyes shrink.