Abnormal, harmful, or undesirable effect on an organism that causes anatomical or functional damage, irreversible physical changes, or increases the susceptibility to other biological, chemical, or environmental stresses.
Optometrists can prescribe glasses, contact lenses, eye exercises, vision aids and therapy, and (in some states) oral and topical medications to treat certain eye diseases. Ophthalmologists are doctors of the eye. They have graduated from medical school and are licensed to practice medicine, so their credentials are either doctor of medicine (MD) or doctor of osteopathy (DO).
Yes, some (if not all) have an eye doctor on-site and will give examinations for glasses or contacts, which you can pick up right after they prescribe you. I switched from my original ophthalmologist to Costco's after going in because their prices were hard to beat and their service was quick and easy.