Chapter 23, Problem 87RPP

BIO Laser surgery for the eye LASIK (laser-assisted in situ keratomileusis) is a surgical procedure intended to reduce a person's dependency on glasses or contact lenses. Laser eye surgery corrects common vision problems, such as myopia (nearsightedness), hyperopia (farsightedness), astigmatism (blurred vision resulting from corneal irregularities), or some combination of these. In myopia, the cornea, the clear covering at the front of the eye, is often too highly curved, causing rays from distant objects to form sharp images in front of the retina (Figure 23.34). LASIK refractive surgery can flatten the cornea so that images of distant objects form on the retina. A knife cuts a flap in the cornea with a hinge left at one end of this flap. The flap is folded back, exposing the middle section of the cornea. Pulses from a computer-controlled laser vaporize a portion of the tissue and the flap is replaced.

In farsighted people, an object held at the normal near point of the eye (about 25-50 cm from the eye) forms an image behind the retina (Figure 23.35). Increasing the curvature of the cornea causes rays from near objects to produce an image on the retina. Conductive keratoplasty (CK) can increase the curvature of the cornea to correct farsightedness. In CK, a tiny probe emits radio waves in a circular pattern on the outside of the cornea. Exposure to radio waves causes shrinking in the ring around the cornea, which acts like a constrictive band, increasing the overall curvature of the cornea.

In Problem 23.86 what should the cornea-lens focal length be in order to form a sharp image of the book on the retina?

$\begin{array}{l}\begin{array}{l}\text{a}\text{. +1}\text{.96 cm}\hfill \\ \text{b}\text{. +2}\text{.10 cm}\hfill \\ \text{c}\text{. +2}\text{.26 cm}\hfill \\ \text{d}\text{. +2}\text{.42 cm}\hfill \end{array}\\ \text{e}\text{. +27}\text{.90 cm}\end{array}$

Your eyeball is 2.10 cm from the cornea to the retina. A book held 30 cm from your eyes produces an image 2.26 cm from the entrance to the image location. What is the focal length of the cornea-lens system?

$\begin{array}{l}\begin{array}{l}\text{a}\text{. +1}\text{.96 cm}\hfill \\ \text{b}\text{. +2}\text{.10 cm}\hfill \\ \text{c}\text{. +2}\text{.26 cm}\hfill \\ \text{d}\text{. +2}\text{.42 cm}\hfill \end{array}\\ \text{e}\text{. +27}\text{.90 cm}\end{array}$