The cornea is the transpartent, dome-shaped lens that covers the front of the eye that covers the iris, pupil, and anterior chamber. Although the cornea is clear and seems to lack substance, it is actually a highly organized group of cells and proteins.   The cornea, with the anterior chamber and lens, refracts light, with the cornea accounting for approximately two-thirds of the eye’s total optical power.

The cornea receives its nourishment from the tears and aqueous humor (a fluid in the anterior portion of the eye) that fills the chamber behind it. The cornea must remain transparent to refract light properly, and the presence of even the tiniest blood vessels can interfere with this process. To see well, all layers of the cornea must be free of any cloudy or opaque areas.

While the cornea contributes most of the eye’s focusing power, its focus is fixed. The curvature of the lens, on the other hand, can be adjusted to “tune” the focus depending upon the object’s distance.

Reference: cornea