Unit Ten: The Nervous System: B. Special Senses

Chapter 49: The Eye: I. Optics of Vision

Guyton and Hall, Textbook of Medical Physiology, 12th edition

Physical Principles of Optics

• Refraction of Light

a. Refractive index of a transparent substance- ratio ofthe velocity of light in air to the velocity of light inthe substance; the refractive index of air is 1.0

b. Refraction of light rays at an interface between twomedia with different refractive indices

Physical Principles of Optics

Fig. 49.1 Light rays entering a glass surface perpendicular to the light rays (A), and a glass surface angulated to the light rays (B)

Physical Principles of Optics

a. When light rays traveling forward in a beam strike aninterface that is perpendicular to the beam, the raysenter the second medium without deviating from their course.

b. If the light rays pass through an angulated interface,the rays bend if the refractive indices of the twomedia are different from each other.

c. The bending of the light at the angulated surface isrefraction.

Physical Principles of Optics

• Application of Refractive Principles to Lenses

a. Convex lens focuses light rays

Fig. 49.2 Bending of light rays at each surface of a convex spherical lens, showing that parallel light rays are focused to a focal point

Physical Principles of Optics

• Application of Refractive Principles to Lenses

b. Concave lens diverges light rays

Fig. 49.3 Bending of light rays at each surface of a concave spherical lens, showing that parallel light rays are diverged

Physical Principles of Optics

• Application of Refractive Principles to Lenses

c. Cylindrical lens bends light rays in only one plane-comparison with spherical lenses

Fig. 49.4 A: point focus of parallel light rays by a cylindrical convex lens. B: line focus of parallel light rays by a cylindrical convex lens

Physical Principles of Optics

• Application of Refractive Principles to Lenses

d. Combination of two cylindrical lenses at right anglesequals a spherical lens

Fig. 49.5 A: Focusing of light from a point source to a line focus by a cylindrical lens. B: two cylindrical convex lenses at right angles to each other

Physical Principles of Optics

• Application of Refractive Principles to Lenses

e. Focal length of a lens-the distance beyond a convexlens at which parallel rays converge to a commonfocal point

Fig. 49.6

Physical Principles of Optics

• Application of Refractive Principles to Lenses

Fig. 49.6

The two upper lenses have the same focal length, but the light rays entering the top lens are parallel, whereasthose entering the middle lens are diverging; the effectof parallel versus diverging rays on the focal distance isshown. The bottom lens has far more refractive powerthan either of the other two lenses (i.e. much shorterfocal length), demonstrating that the stronger the lensis, the nearer to the lens the point focus is.

Physical Principles of Optics

• Application of Refractive Principles to Lenses

f. Formation of an image by a convex lens

Fig. 49.7 A: two point sources of light focused at two separate points on opposite sides of the lens. B: Formation of an image by a convex spherical lens

Physical Principles of Optics

• Application of Refractive Principles to Lenses

g. Measurement of the refractive power of a lens—”diopter”

The more a lens bend light rays, the greater itsrefractive index. Refractive power is measured indiopters

Fig. 49.8 Effect of lens strength on the focal distance

Optics of the Eye

• The Eye as a Camera

Fig. 49.9 The eye as a camera. The numbers are refractive indices.

Optics of the Eye

• The Eye as a Camera- lens system of the eye has fourrefractive interfaces:

a. The interface between the air and the anterior surface of the cornea.

b. The interface between the posterior surface of the cornea and the aqueous humor.

c. The interface between the aqueous humor and theanterior surface of the lens.

d. The interface between the posterior surface of the lensand the vitreous humor.

Optics of the Eye

• Formation of the Image on the Retina

a. Image is inverted

b. Image is reversed with respect to the object

• Mechanism of Accommodation- in children therefractive power of the lens can be increasedvoluntarily from 20 to 34 diopters (increase of 14diapters); allows the lens to change from moderatelyconvex to greatly convex; allows the lens and corneato bend light rays when viewing objects up close

Optics of the Eye

• Accommodation is Controlled by ParasympatheticNerves

• Presbyopia- loss of accommodation by the lens; asyou age the lens loses it elasticity and becomes larger and thicker; by the age of 45-50, diopters drop from 14 to 2 and by 70 it is 0.

• Pupillary Diameter- major function of the iris is toincrease the amount of light that enters the eye during darkness and decrease the amount of light that enters in daylight

Optics of the Eye

Fig. 49.10 Mechanism of accommodation (focusing)

Optics of the Eye

Fig. 49.11 Effect of small (top) and large (bottom) pupillary apertures on the depth of focus

• “Depth of Focus” of the Lens Increases withDecreasing Pupillary Diameter

Optics of the Eye

• Errors of Refraction

a. Emmetropia (Normal Vision)

b. Hyperopia (Farsightedness)

c. Myopia (Nearsightedness)

d. Correction of hyperopia and myopia withlenses

Optics of the Eye

Fig. 49.12 Parallel light rays focus on the retina in emmetropia, behind the retina in hyperopia, and in front of the retina in myopia

Fig. 49.13 Correction of myopia with a concave lens, correction of hyperopia with a convex lens

Optics of the Eye

• Errors of Refraction (cont.)

f. Astigmatism-refractive error that causes the visual image in one plane to focus at a differentdistance from that of the plane at right angles;

Results from too great a curvature in the corneaor lens; accommodation of the lens cannot compensate because each plane requires accommodation

g. Astigmatism is corrected with two cylindrical lensesof different strengths at right angles

Optics of the Eye

• Visual Acuity

Fig. 49.16 Maximum visual acuity for two-point sources of light

Optics of the Eye

• Visual Acuity- at the fovea centralis of the macula

• Depth Perception-normally perceives distance bythree major means:

a. Sizes of the images of known objects on the retinab. The phenomenon of moving parallaxc. The phenomenon of stereopsis (binocular vision)

Optics of the Eye

Fig. 49.17 Perception of distance by the size of the image and stereopsis

• Depth Perception

Optics of the Eye

• Fluid System of the Eye

a. Aqueous humor-formed by the ciliary body; flowsthrough the pupil into the anterior chamberfinally entering the canal of Schlemm

b. Intraocular pressure is about 15 mm Hg (range of12-20 mm Hg)

c. Glaucoma-pressure rises above 25 mm and canget as high as 60-70 mm; one of the most commoncauses of blindness

Optics of the Eye

Fig. 49.19 Formation and flow of fluid in the eye Fig. 49.20 Anatomy of the ciliary processes; aqueous humor is formed on surfaces