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 of
the velocity of light in air to the velocity of light in
the substance; the refractive index of air is 1.0
b. Refraction of light rays at an interface between two
media 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 an
interface that is perpendicular to the beam, the rays
enter 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 two
media are different from each other.
c. The bending of the light at the angulated surface is
refraction.
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 planecomparison 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 angles
equals 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 convex
lens at which parallel rays converge to a common
focal 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, whereas
those entering the middle lens are diverging; the effect
of parallel versus diverging rays on the focal distance is
shown. The bottom lens has far more refractive power
than either of the other two lenses (i.e. much shorter
focal length), demonstrating that the stronger the lens
is, 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 its
refractive index. Refractive power is measured in
diopters
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 four
refractive 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 the
anterior surface of the lens.
d. The interface between the posterior surface of the lens
and 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 the
refractive power of the lens can be increased
voluntarily from 20 to 34 diopters (increase of 14
diapters); allows the lens to change from moderately
convex to greatly convex; allows the lens and cornea
to bend light rays when viewing objects up close
Optics of the Eye
• Accommodation is Controlled by Parasympathetic
Nerves
• Presbyopia- loss of accommodation by the lens; as
you 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 to
increase 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
• “Depth of Focus” of the Lens Increases with
Decreasing Pupillary Diameter
Fig. 49.11 Effect of small (top) and large (bottom) pupillary apertures
on the depth of focus
Optics of the Eye
• Errors of Refraction
a. Emmetropia (Normal Vision)
b. Hyperopia (Farsightedness)
c. Myopia (Nearsightedness)
d. Correction of hyperopia and myopia with
lenses
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 different
distance from that of the plane at right angles;
Results from too great a curvature in the cornea
or lens; accommodation of the lens cannot
compensate because each plane requires
accommodation
g. Astigmatism is corrected with two cylindrical lenses
of 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 by
three major means:
a. Sizes of the images of known objects on the retina
b. The phenomenon of moving parallax
c. The phenomenon of stereopsis (binocular vision)
Optics of the Eye
• Depth Perception
Fig. 49.17 Perception of distance by the size of the image and stereopsis
Optics of the Eye
• Fluid System of the Eye
a. Aqueous humor-formed by the ciliary body; flows
through the pupil into the anterior chamber
finally entering the canal of Schlemm
b. Intraocular pressure is about 15 mm Hg (range of
12-20 mm Hg)
c. Glaucoma-pressure rises above 25 mm and can
get as high as 60-70 mm; one of the most common
causes 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