So the pupil is the opening of the eye. The pupil is the opening that actually allows light to pass through this region and into this large cavity that contains another type of gellike fluid known as our vitreous humor. So what exactly is this pupil? So the pupil is this opening that can actually be controlled by special types of smooth muscles. So these smooth muscles are known as the iris. So when we look into the mirror, the iris is the colored portion of the eye, while the square is the white portion of the eye.