Human Eye Anatomy and Function

• The lecture covers the physics of eyes and vision.
• Visual system, vision elements, focusing elements, sensitivity, image formation, and vision defects are covered.

Visual System

• The visual system constitutes the sense of vision and has three major components.
• The eyes focus an image from the outside world onto the light-sensitive retina.
• Millions of nerves carry information deep into the brain.
• The visual cortex is the part of the brain where information is processed.
• Blindness can occur if any of these parts do not function properly.

Vision Elements of the Eye

• The cornea is the transparent part on the front of the eye that provides approximately two-thirds of the eye's focusing power.
• The eye consists of two focusing elements, cornea and lens.
• The cornea is a fixed-focus element.
• The lens is variable in shape, enabling the eye to focus on objects at varying distances.
• The cornea focuses light by bending it and the amount of bending depends on the curvatures and the speed of light through it.
• The index of refraction is nearly constant for all corneas but the curvature varies among individuals, leading to defective vision.
• The iris adapts vision from light to dark and vice versa and increasing or decreasing retina incident light.
• Additionally, under bright light conditions it plays a role in reducing lens defects.
• The pupil is the small opening in the iris's center where light enters the lens.
• The pupil appears black because the light is absorbed inside the eye.
• The pupil averages 4mm, varying from 3mm in bright light, and 8mm in dim light although the physiologic reasons are unclear.
• The lens is variable in shape, focusing at different distances, more curved in the back than the front.
• The lens’ focusing power is less than cornea at 1/3, due to surrounding substances having similar refraction indexes.
• The lens is made up of layers which don't share the same refractive index.
• The aqueous humor fills the gap between lens and cornea and mostly consists of water with a refractive index ~1.33.
• The aqueous humor is continuously produced and any surplus escapes through the Canal of Schlemmt.
• Blockage results in increased pressure/ glaucoma, while the eye maintain 20mmHg internal pressure thanks to it.
• In addition, it provides nutrients to the non-vascularized cornea and lens.
• The vitreous humor is a clear, jelly-like substance that fills the larger space between the lens and the retina, keeping the eye shape fixed.
• The sclera is a tough, white, light-tight covering over the eye, except for the cornea, and it's protected by the conjunctiva transparent coating.
• The retina is the light-sensitive portion of the eye, converting light images into electrical nerve impulses.

Sensitivity of the Eye and Image Formation on the Retina

• The retina is the light detector, converting light into electrical nerve impulses and absorption causing a photochemical reaction.
• Most vision is restricted to the macula lutea (yellow spot) and all detailed vision takes place in the fovea centralis (0.3mm in diameter).
• Two types of photoreceptors in the retina are cones and rods.
• Cones and rods show symmetrical distribution except in region of blind spot.

Cones Facts

• Each eye has 6.5 million.
• Used for daylight to see fine details and recognize different colors.
• Primarily found in the fovea centralis.
• Each has its own nervous link to the brain.
• Density affects the amount of detail.
• Maximum sensitivity is about 550 nm in the yellow-green region.
• Rapid dark adaptation in 5 minutes.

Rods Facts

• Each eye has 120 million.
• They are used for night vision and peripheral vision.
• Covers retina except for the maximum density at about 20° from the vision axis.
• Hundreds of rods send information to the same nerve fiber.
• Their ability to resolve two close light sources is poor.
• Most sensitive to the blue-green light (~510nm).
• Continue to dark adapt for 30 to 60 min.

Image Formation on the Retina

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

Near and Far Points

• A person with normal vision has a typical near point of 25cm and a typical far point at infinity.
• The focal length of the eye lens is calculated using the formula 1/f = 1/do + 1/di.

Accommodation

• Eye lens strength in terms of optical power (P): P (diopter) = 1/f(m).
• Convex lens power is positive, diverging lens is negative.
• At near point Pnear = (1/0.25) + (1/0.02) = 54 m-1 = 54 D.
• At far point: Pfar = (1/∞) + (1/0.02) = 50 m-1 = 50 D.
• Paccomm. = [P]near - [P]far = [(1/do) + (1/di)]near - [(1/do) + (1/di)]far.

Diopter Strength of the Eye

• With focal lengths F1 and F2, use 1/F = 1/F1 + 1/F2 + ...+ 1/Fn.
• Assume lens A with focal length FA = 0.33m is combined with lens B with focal length FB = 0.25m: 1/F = 1/0.33 + 1/0.25 = 3 + 4 = 7 D.

Vision Defects

• Nearsightedness (myopia) is when near objects are clear, but distant objects are blurry due to too strong lens or too long eyeball.
• The eye over converges near parallel lights and cross in front of retina.
• Vision correction for nearsightedness consists of placing a diverging eyeglass (concave) lens in front of the eye.
• Farsightedness (hyperopia) is clear far vision, blurry distant/ is caused by a weak lens or short eyeball.
• Corrected with a converging eyeglass (convex) lens.
• Presbyopia is the loss of the ability to focus on near objects due to aging, which starts around age 45 and requires bifocal lenses.
• Bifocal lenses contains both concave and convex lenses and is easily corrected.
• The upper portion consists of a concave lens facilitating distant vision, lower part a convex lens to facilitate near vision.
• Astigmatism vision defect are not clear.
• Astigmatism is corrected with asymmetric lens.
• The equation Pnormal = Ppatient + Pglass can be used to find the power of needed glass.

Explore the intricate functions of the human eye, from the iris controlling light to the cornea's role in focusing. Understand how different parts of the eye contribute to vision and potential problems that may arise. Damage to the visual cortex can affect sight.