Pupil of eye. Artwork showing how the pupil of the eye reacts in three different light intensities. The pupil is the circular opening (black) in the centre of the iris (blue). The muscles of the iris widen or narrow the pupil to adjust the amount of light entering the eye. In dim light at top, the pupil has dilated (widened) to allow more light to reach the light-sensitive retina at the back of the eye (not seen). In brighter light (centre) the pupil is smaller. Very bright conditions cause the pupil to constrict (narrow) to minimize the amount of light hitting the retina.