Anatomy and Function of the Eye

• The lecture covers the physics of eyes and vision
• Main topics include the visual system, vision elements of the eye, focusing elements, sensitivity, retinal image formation, and vision defects.

Visual System

• Vision involves the eyes, a system of nerves, and the visual cortex
• The eyes focus images onto the retina
• Nerves carry information to the brain
• The visual cortex processes information
• Blindness results if any of these parts malfunction

Vision Elements of the Eye

• Includes the cornea, iris, pupil, lens, aqueous humor, vitreous humor, sclera, and retina

Cornea

• It is a clear, transparent bump on the front of the eye
• Contributes about two-thirds of the eye's focusing power
• Is a fixed focusing element

Lens

• It is a variable shape that can focus objects at varying distances

Focusing Elements Refraction

• The cornea focuses by bending (refracting) light rays
• The index of refraction is nearly constant across corneas
• Curvature variations can lead to defective vision
• Refraction indexes: 1.37 (cornea), 1.33 (aqueous humor and vitreous humor), 1.38 (lens cover), and 1.41 (lens center)

Iris

• Is the colored part of the front eye
• Adapts vision from light to dark and vice versa
• Regulates light incident on the retina based on adaptation needs
• Plays a role in reducing lens defects under bright light

Pupil

• Is a small opening in the center of the iris where light enters the lens
• Appears black due to light absorption inside the eye
• Averages about ≈ 4mm in normal light
• Can change from ≈ 3mm in bright light to 8mm in dim light

Lens Qualities

• It varies in shape with front and back surfaces focusing objects at various distances
• Curves more in the back than the front
• Has less focusing power than the cornea (1/3) because it’s surrounded by substances of similar refractive indices
• Layers are similar to an onion but the layers don't all have the same n

Aqueous Humor

• Fills the space between the lens and the cornea
• Is mostly water with n ≈1.33
• It is continuously produced, and surplus escapes through the Canal of Schlemm
• Blockage can cause increased eye pressure and glaucoma
• Maintains internal eye pressure at around 20mm Hg
• Contains blood components and supplies nutrients to the cornea and lens

Vitreous Humor and Sclera

• Vitreous humor is a jelly-like substance filling the space between the lens and retina that helps maintain the fixed shape
• Sclera is a robust covering coating eye except cornea
• It is protected with the transparent conjunctiva

Retina

• Is the eye's light-sensitive part
• Converts light images into electrical nerve impulses

Eye Image Formation on the Retina

• The retina is the light detector, converting light to electrical impulses
• Light absorption by photoreceptors initiates a photochemical reaction that produces electrical nerve impulses
• Most vision relies on the macula lutea, or yellow spot
• Detailed vision occurs in the fovea centralis (0.3 mm diameter) within the yellow spot

Eye Photoreceptors

• Types include cones and rods that are symmetrically distributed, except in the blind spot, where there are neither
• Cones: 6.5 million, daylight/color vision, found in fovea centralis, connected to the brain, high detail resolution, high sensitivity at 550 nm, rapid dark adaptation
• Rods: 120 million, night/peripheral vision, found mostly in the retina (20°angle), share nerve fiber, low light resolution, high sensitivity to blue-green light (~510nm), and adapt to dark in (30 to 60 min)

Image Formation Details

• Near Objects: eyes muscles tighten, muscle fibers lessen which cause eye lens to thicken and power increases
• Far objects: Eye muscles relax, muscle fibers get longer to eye lens becomes thin reducing power

Accommodation and Near/ Far Points

• A person with normal vision: typical near point is 25cm and a typical far point is infinity
• The focal length of the eye: 1/f = 1/do + 1/di, where d₁ = 2 cm = 0.02 m
• The strength of the eye lens can be is expressed in term of the optical power (P): P (diopter) = 1/ f(m)

Vision Defects

• Nearsightedness (Myopia): near objects are clear and far objects are blurry
• Arises from a too-strong eye lens or too-long eyeball, causing converges before retina
• Corrected with a diverging eyeglass (concave lens) in front of the eye
• Farsightedness (Hyperopia): the ability to see far objects clearly whereas distant objects are blurry
• The lens is too weak, or the eyeball is too short, so a close object converges after retina
• Corrected with a converging eyeglass (convex lens) in front of the eye
• Presbyopia: loss of near-vision focus due to aging around 45 is not a disease
• Corrected with Bifocal lens is like a common lens that has a concave lens top for distance and convex lens bottom aids with near vision
• Astigmatism happens when point images don't form clearly on the retina because the cornea curvatures are unequal
• Often corrected with asymmetric lens
• How to correct a vision defect: start with Pnormal = Ppatient + Pglass
• To find the total diopter strength of the eye: 1/F = 1/F1 + 1/F2

Explore the anatomy of the eye and its functional components. Understand the roles of the cornea, lens, aqueous humor, vitreous humor, retina, and iris. Learn about vision, focusing, and potential eye conditions.