The problem
Image forms behind retina
A solution
Corrected with converging lens
33.12 - Physics at work: laser eye surgery
Laser eye surgery, a procedure now quite popular, provides a way to alter the shape of
the cornea so that glasses or contact lenses are not needed, or are required less
frequently. It has proven quite popular amongst both celebrities (such as Tiger Woods)
and non-celebrities.
To explain how the procedure works, we first review some fundamentals of how the eye
focuses an image. The eye has two components that are most responsible for the
location of the image it creates: the cornea (its outermost surface) and an internal
variable lens. Light refracts as it passes through both of these components.
Although one might think most of the refraction occurs at the lens, in fact about two-
thirds of the refraction occurs at the cornea. It occurs there because the index of
refraction of the cornea differs substantially from that of air. This difference is much
larger than differences in the indices of refraction within the eye. The cornea and the
lens combine to focus the light at the retina, which is at the back of the eye. The eye
adjusts the curvature of the lens in order to alter its focal length, enabling it to focus
objects at various distances. When the eye can change the lens shape enough so that
light converges crisply at the retina for objects both near and far, a person has no need
for glasses.
However, many people need glasses or contact lenses in order to see clearly.
Nearsightedness and farsightedness occur when the lens’s shape cannot be changed
enough to sharply focus images on the retina. Eyeglasses and contact lenses
compensate for this by providing a second lens that assists in the process of focusing
the image at the proper position.
Laser eye surgery takes a different tack: It changes the eye itself. The surgery changes
the shape of the cornea in order to correct for the limitations of the lens. The procedure
does not alter the lens inside the eye; rather, it “tunes” the cornea.
Currently, a type of laser surgery called LASIK (laser insitu keratomileusis) is the most
widely used. To begin the process, medical personnel first determine how the cornea’s
curvature must be modified in order to improve vision. Then, a surgeon temporarily
peels back the epithelium (the thin outermost layer of the eye) and trains a laser on the
eye to remove small amounts of the cornea. The epithelium is then put back to cover
the eye’s surface.
Laser surgery can be used to treat both nearsightedness and farsightedness. Nearsightedness occurs when the lens cannot become
sufficiently flat, resulting in an image formed in front of the retina. This is shown in Concept 1. To compensate for this, the surgeon flattens the
The problem: nearsightedness
Image forms in front of retina
The solution
Cornea flattened to correct vision
After surgery í image forms on retina