How do scars develop on corneas?

From deeper injuries: A scar can form on a cornea after a corneal ulcer or corneal erosion, because the injury goes deeper, beyond the surface layer of cells. A corneal abrasion injures just the top layer of cells, and usually heals in 1-2 days without a scar. For the deeper injuries, sometimes steroid eye drops are used later in the healing period to help decrease scar formation.

Corneal scars: Like any scar, corneal scars come from an injury to the cornea- either a cut, rubbing the eyes or an infection(often from unclean contacts) or even from dry eyes. If the eye is too dry, the cornea will be damaged and can scar. Protect your eyes by wearing glasses, following instructions with contacts and not putting fingers or objects in eyes. Glasses with uv protection are best.