Dr. Najeeb’s lectures on the Structure and Function of the Human Eye.
Pupil: “The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina. It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye that mostly miss exiting the narrow pupil.”
Iris: The iris is located behind the cornea. It acts as a diaphragm, like a shutter on a camera, regulating the amount of light that enters through the pupil. The pupil appears as a black hole at the center of the iris. The pupil is the opening through which light enters the eye.
Conjunctiva: “The conjunctiva is the membrane that lines the eyelid and loops back to cover the sclera (the tough white fiber layer covering the eye), right up to the edge of the cornea (the clear layer in front of the iris and pupil—see Structure and Function of the Eyes). The conjunctiva helps protect the eye by keeping small foreign objects and infection-causing microorganisms out and by contributing to the maintenance of the tear film.”
“Near the front of the eye, in the area protected by the eyelids, the sclera is covered by a thin, transparent membrane (conjunctiva), which runs to the edge of the cornea. The conjunctiva also covers the moist back surface of the eyelids and eyeballs.”
In other words, the conjunctiva is a thin transparent membrane that covers the sclera. The sclera is the white part of the eye that surrounds the cornea and is continuous with it.
Reference
The above images are from the National Eye Institute
https://nei.nih.gov/photo/anatomy-of-eye