The colored part of the eye is called the iris. It is composed of two muscles. The constrictor and the dilator. The black circle at the center of the iris is called the pupil and is actually just an opening to allow light into the eye. The pupil appears black because the inside of the eye contains pigment to absorb all of the light that enters it, not allowing any of it to reflect back out.
When light enters your eye there is a muscular reflex in your iris to regulate the amount of light that hits the retina. Too much light and constrict muscles tighten to making your pupil smaller to limit the amount of light entering your eye. Not enough light and the dilator muscles contract allowing your pupil to get bigger.
During a comprehension eye exam, your eye doctor might want to dilate your pupils to evaluate the back of each eye under high magnification. Without dilation, the bright exam light automatically causes your pupils to constrict, making a much smaller opening to look through.
Dilating drops cause your pupils to enlarge by inhibiting the constrictor muscles from reacting to light., then the doctor can use a bright exam light combine with a magnification lens for an unobstructed view of the internal structure of each eye, including the retina, macula and optic discs.
Dilation drops typically cause blurred near-vision and light sensitivity lasting about 4 to 6 hours. Sunglasses are usually worn afterward to relieve glare discomfort, but you should be safe to drive with caution.