Notes:

The optical system of the eye is not very complicated, as optical systems go. It has two refracting elements that focus light rays entering the eye to form a retinal image.

The first element is the cornea, which is the transparent part of the eye's globe. It is made from the same kind of tissue that the sclera, which is the white part of the globe, is made from. There are subtle difference between sclera and cornea at the microscopic level that makes the sclera opaque but leaves the cornea clear and transparent so that it can act as an optical lens. The optical focusing power of the cornea is fixed and is not great enough to form a focused retinal image by itself.

The extra optical power needed to cast a clear retinal image comes from a second lens located inside the eye, just behind the iris as shown in this cross-sectional view. Unlike the cornea, the internal lens of the eye is adjustable so that we can focus on objects which are at different distances from the eye.

The iris is an opaque muscle that controls the amount of light entering the eye and which gives the eye its blue, green, or brown color. The pupil is a hole in the iris that allows light into the eye. The reason the pupil looks black is because almost all of the light that enters the eye gets absorbed, which means very little light is available for reflecting back out of the eye for somebody else to see.