eyes

Refraction

The bending of light rays at an angulated interface

Concave Lens

Diverges light rays

Convex Lens

Converges light rays

Diopter

Measurement of the refractive power of a lens

Refractive Index

The ratio of the velocity of light in air to the velocity in the substance

1.00

The refractive index of air

1.50

The refractive index of glass if light travels through it at a velocity of 200,000 km/sec

Four

Number of refractive interfaces in the lens system of the eye.  

Air and the anterior surface of the cornea

Posterior surface of the cornea and the aqueous humor

Second refractive interface of the eye

Aqueous humor and the anterior surface of the lens

Third refractive interface of the eye

Posterior surface of the lens and the vitreous humor

Fourth refractive interface of the eye

1.38

Refractive index of the cornea

1.33

Refractive index of the aqueous humor

1.40

Refractive index of the crystalline lens (average)

1.34

Refractive index of the vitreous humor

Anterior surface of the cornea

Provides about two-thirds of the eye's refractive power

Inverted and reversed

Orientation of the image on the retina

Brain

Perceives objects in the upright position despite the upside-down orientation on the retina

Accommodation

The process by which the shape of the lens is changed

70

Approximate number of suspensory ligaments that attach radially around the lens

Ciliary muscle

Controlled by parasympathetic nerve signals

Decreasing pupillary diameter

Increases the depth of focus of the lens system

Emmetropia

Normal vision

Ciliary muscle

Must contract to focus objects at close range

Hyperopia

Farsightedness

Too short eyeball or too weak lens system

Causes of hyperopia

Presbyopic

Term for a lens that has lost its ability to accommodate, often occurring in old age

Myopia

Nearsightedness

Too long eyeball or too much refractive power in the lens system

Causes of myopia

Concave spherical lens

Used to correct myopia

Convex lens

Used to correct hyperopia

Astigmatism

A refractive error causing the visual image in one plane to focus at a different distance from the plane at right angles

Too great a curvature of the cornea in one plane

Most common cause of astigmatism

Cylindrical lens

Used to correct astigmatism

Keratoconus

A condition with an odd-shaped, bulging cornea

Tear fluid

Holds contact lenses in place

Anterior surface of the cornea

Refraction nullified almost entirely by contact lens

Outer surface of the contact lens

Plays the major role in refraction when using contact lenses

Cataract

A cloudy or opaque area in the lens

Denatured proteins

Cause of cataracts in the early stage

Surgical removal of the lens

Correction for cataracts

Sclera, choroid, retina

Coats of the eye

Retina

The inner coat of the eye

Fovea

A minute area in the center of the retina for acute and detailed vision

Cones

Compose the central fovea

25 seconds of arc

Normal visual acuity of the human eye for discriminating between point sources of light

5 to 2 millimeters

Distance at which a person with normal visual acuity can barely distinguish two bright pinpoint spots of light 10 meters away

5 millimeter (500 micrometers)

Diameter of the fovea

2 degrees

Visual field where maximum visual acuity occurs

10-fold

Decrease in visual acuity as the periphery of the retina is approached

Cones and Rods

Photoreceptors in the retina

Cones

Responsible for color vision

3 million

Number of cones in the retina

Rods

Responsible for vision in the dark

100 million

Number of rods in the retina

Nine

Number of layers in the retina (from outside to inside)

Pigmented layer

Outermost layer of the retina

Melanin

Black pigment in the pigment layer

Albinos

People who lack melanin pigment

Vitamin A

Stored in large quantities in the pigment layer

Rhodopsin

Light-sensitive pigment in the outer segment of the rod.  

Scotopsin and retinal

11-cis retinal

Particular type of retinal that binds with scotopsin

Photoactivation of electrons

Causes rhodopsin to decompose when light energy is absorbed

All-trans retinal

Formed when rhodopsin decomposes

Bathorhodopsin

The immediate product of rhodopsin decomposition.  

Lumirhodopsin, Metarhodopsin I, Metarhodopsin II

Activated rhodopsin

Another name for metarhodopsin II

Retinal isomerase

Enzyme that catalyzes the conversion of all-trans retinal into 11-cis retinal

Vitamin A

All-trans retinol

Night blindness

Occurs with severe vitamin A deficiency

Hyperpolarization

The change in the intrarod membrane potential when the rod is exposed to light

Decreased rod membrane conductance for sodium ions

Cause of hyperpolarization

Transducin

Activated by Metarhodopsin II

Phosphodiesterase

Activated by Transducin

cGMP

Hydrolyzed by phosphodiesterase

Rhodopsin kinase

Inactivates activated rhodopsin

Light adaptation

Decrease in photoreceptors and sensitivity of eye to light with prolonged light exposure

Dark adaptation

Increase in light-sensitive pigments in rods and cones with prolonged dark exposure

Red, green, and blue

Monochromatic lights that the human eye can detect to perceive almost all gradations of color

580 nanometers

Wavelength of orange monochromatic light

450 nanometers

Wavelength of blue monochromatic light

99:42:0

Ratios of stimulation of red, green, and blue cones by orange light

0:0:97

Ratios of stimulation of red, green, and blue cones by blue light

83:83:0

Ratios of stimulation interpreted as yellow

31:67:36

Ratios of stimulation interpreted as green

White

Sensation from about equal stimulation of all red, green, and blue cones

Red-green color blindness

Inability to distinguish red from green

Protanope

Person with loss of red cones

Deuteranope

Color-blind person who lacks green cones

X chromosome

Location of genes that code for cones

Mother

Parent who passes color blindness to son

8 percent

Percentage of women who are color blindness carriers

Ishihara charts

Spot charts used to determine color blindness

74

Number read by a person with normal color vision on an Ishihara chart

21

Number read by a red-green color-blind person on an Ishihara chart

42

Number read by a person with normal color vision on a second Ishihara chart

2

Number read by a red-blind person on the second Ishihara chart.  

4

Optic chiasm

Where optic nerve fibers from the nasal halves of the retinas cross to the opposite sides

Optic tracts

Formed by the joining of fibers from the nasal halves of the retinas and the opposite temporal retinas

Dorsal lateral geniculate nucleus of the thalamus

Where fibers of each optic tract synapse

Geniculocalcarine fibers

Pass from the thalamus to the primary visual cortex