What is the Iris?
The iris is the coloured part of the eye and has a circular hole in the middle called the pupil. The iris is effectively made up of two muscles; one makes the pupil smaller and the other makes it larger. When dilating drops are put in the eye to dilate the pupil they act on one or both of theses muscles, blocking the first and activating the second.
The iris controls the amount of light that enters the eye. The pupil constricts in bright lighting so that there’s no glare and dilates in the dark to let in as much light as possible.