6.1.1 Refraction in Ophthalmology.
Refraction is of biophotonics interest when dealing with transparent tissues such as
certain ophthalmological material (e.g., eye materials of the cornea, lens, vitreous
humor, and aqueous humor) [ 12 – 14 ]. As Fig.6.2shows, the human eye has a series
of refractive media that focus the image of objects onto the retina. The cornea is the
major optical medium in the eye because, in addition to having a large radius of
curvature in the central area, it has a high refractive index of 1.376 compared to
1.00 for air. The crystalline lens has a refractive index of 1.406 at its center, and can
change its shape dynamically to allow the eye to focus on objects located at various
distances. Eye treatments involving considerations of the refractive index include
using excimer (UV) lasers for surgical remodeling of the cornea to correct vision
disorders and for cataract surgery to replace clouded lenses. The goals of refractive
eye surgery are to treat degenerative eye disorders like keratoconus (a thinning
condition of the cornea that causes visual distortion) and to alleviate or cure
common vision disorders such as myopia (nearsightedness), hyperopia (farsight-
edness), and astigmatism.
Example 6.1Consider a visible light beam of parallel light rays that
impinges on a cornea, as shown in Fig.6.2. Suppose the outer ray hits the
cornea curvature at an angleθ 1 = 20° with respect to the surface normal. If
the indices of refraction of air and the cornea are 1.000 and 1.376, respec-
tively, what is the angleθ 2 at which the light from this ray enters the aqueous
humor area behind the cornea?
Solution: From Snell’s law as given by Eq. (2.22) the refracted angle ish 2 ¼sin^1 ½ð 1 :000 sin 20Þ= 1 : 376 ¼ 14 : 4 Thus the ray is bent toward the normal and then travels to the lens where it is
focused onto the retina along with the other entering light rays.The discussions in Sect.6.5on photoablation, plasma-induced ablation, and
photodisruption light-tissue interactions describe further procedures and examples
of eye surgery applications.
6.1.2 Specular Reflection
The amount of light that is reflected by an object depends strongly on the texture of
the reflecting surface. Reflection from smooth surfaces such as mirrors, polished
150 6 Light-Tissue Interactions