The Human Visual System

• The lecture discusses the physics of eyes and vision.

Visual System

• Vision consists of the eyes, nerves, and the visual cortex.
• The eyes focus images on the retina.
• Nerves carry information to the brain.
• The visual cortex in the brain processes information.
• Blindness results from any part of the system malfunctioning.

Vision Elements of the Eye

• The cornea bends light, doing about two-thirds of the focusing.
• The eye has two main focusing elements: the cornea (fixed) and the lens (variable).
• The lens can adjust to focus objects at various distances.
• The iris adapts vision from light to dark conditions.
• The iris helps the eye by regulating incident light on the retina and reducing lens defects in bright light.
• The pupil is the opening in the iris where light enters the lens.
• The pupil's diameter ranges from approximately 3mm in bright light to 8mm in dim light.
• The lens' shape is variable, focusing objects at different distances with its front and back surfaces.
• The lens is more curved in the back than in the front.
• The lens' focusing power is less (1/3) than the cornea.
• The lens is made of layers with varying refractive indexes.
• The aqueous humor fills the space between the lens and cornea with a water-like fluid.
• The aqueous humor has a refractive index of about 1.33.
• It is continuously produced and escapes through the Canal of Schlemm.
• Blockage of the drain tubes results in glaucoma.
• It maintains internal eye pressure (20mm Hg) and provides nutrients.
• The vitreous humor is clear and jelly-like, filling the space between the lens and retina, fixing eye shape.
• The sclera is the tough, white covering of the eye, protected by the conjunctiva.
• The retina converts light into electrical nerve impulses sent to the brain.

Focusing Elements of the Eye

• The cornea bends light based on surface curvature and light speed compared to its surroundings.
• The index of refraction is nearly constant for all corneas.
• Curvature varies, causing defective vision

Refraction Indexes

• Cornea: 1.37
• Aqueous Humor: 1.33
• Lens Cover: 1.38
• Lens Center: 1.41
• Vitreous Humor: 1.33

Sensitivity of the Eye and Image Formation on the Retina

• The retina converts light images into electrical nerve impulses.
• Absorption of a photon by a photoreceptor causes a photochemical reaction.
• Most vision depends on the macula lutea (yellow spot).
• Detailed vision occurs in the fovea centralis (0.3mm diameter).

Eye Photoreceptors

• Two types of photoreceptors are in the retina: cones and rods.
• Rods and cones are distributed symmetrically except in the blind spot.
• The blind spot has neither rods nor cones.
• The blind spot region is from about 13° to 18°.
• The cones are 6.5 million in each eye.
• The cones are for daylight and color vision.
• The cones are primarily found in the fovea centralis.
• Each cone has its own nervous link to the brain.
• Cone density affects the amount of detail resolved.
• Cones possess a maximum sensitivity at about 550 nm (yellow-green).
• Cones adapt rapidly to darkness (5min).
• The rods are 120 million in each eye.
• The rods are for night and peripheral vision.
• The rods cover most of the retina, with max density at about 20°from the vision axis.
• Hundreds of rods send information to the same nerve fiber.
• Rods' ability to resolve close light sources is poor.
• Rods are most sensitive to blue-green light (~510nm).
• Rods continue to adapt to darkness for 30-60 min.

Image Formation on the Retina

• For near objects, eye muscles tauten, fibers shorten and eye lens becomes thicker and more powerful.
• For far objects, the eye muscles relax, fibers lengthen, and the eye lens becomes thin and less powerful.

Accommodation & Near and Far Points

• A person with normal vision typically has a near point (do) of 25cm and a far point (do) of infinity.
• Accommodation is expressed by optical power (P)= 1/f(m)
• Convex lens (convergence) is positive, and concave lens (divergence) is negative.

Normal Vision

• At near point, P(near) = 54D.
• At far point, P(far) = 50D.
• Diopter strength of the eye, 1/F = 1/F1 + 1/F2 ...+ 1/Fn

Vision Defects - Nearsightedness (Myopia)

• Clear vision of near objects, blurry vision of distant objects.
• Due to too strong an eye lens or too long an eye ball.
• The eye converges rays excessively, crossing in front of the retina.
• Corrected by placing a diverging (concave) lens in front of the eye.

Farsightedness (Hyperopia)

• Clear vision of far objects but with near objects appearing blurry.
• May develop due to a weak eye lens or a short eye ball.
• Eye unable to converge rays sufficiently to meet on retina
• Corrected with a converging (convex) lens in front of the eye.

Presbyopia

• It is the loss of the eye's ability to change its focus to see objects that are near.
• This condition is a part of the natural aging process.
• Symptoms appear around age 45.
• Requires Bifocal lens: concave and convex lenses. Concave for distant and convex facilitates near vision.

Astigmatism

• It is the inability to form clear point images on the retina because of unequal curvature in different directions
• Corrected with asymmetric lens

Vision Defects - Corrective Lenses

• The power of corrective glass is calculated by: P(normal) = P(patient) + P(glass)

Explore the components and functions of the human visual system. This includes the cornea, lens, pupil, retina, and visual cortex. Understand how these parts work together to enable sight.