Anatomy and Function of the Eye

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

Visual System

• This system consists of the eyes, millions of nerves, and the visual cortex.
• Eyes focus images on the light-sensitive retina from the outside world.
• Millions of nerves carry information to the brain.
• The visual cortex in the brain processes the information.
• If any of these components fail, blindness results.

Vision Elements of the Eye

• The cornea is a transparent bump on the front of the eye, responsible for about two-thirds of the eye's focusing power.
• The cornea is a fixed focus element.
• The lens shape is variable, allowing focus at different distances
• The Iris is colored and located at the front.
• The iris adjusts vision in bright and low light conditions.
• It aids the eye by aiding the light reaching the retina.
• Under light conditions, it will reduce defects of the lens.
• The pupil is the small opening in the iris's center, which allows light into the lens.
• When no light is present it appears black because light is absorbed inside the eye.
• Under average lighting conditions, the opening is about 4mm.
• Bright light will cause the opening to be ≈ 3mm, and dim light can expand it to 8mm.
• The lens shape is changeable and focuses at various distances with its front and back surfaces.
• The lens is more curved in the back compared to the front.
• Its focusing power is one-third of the cornea’s because it is surrounded by indexes of refraction.
• The Aqueous humor is the space between the cornea and the lens filled with watery fluid (n ≈ 1.33).
• It is continuously produced, excess escapes through the "Canal of Schlemm."
• Blockage of the drain tubes results to increased pressure in the eye that is Glaucoma.
• Eye internal pressure is maintained around 20mm Hg.
• Blood components and nutrients are provided to the non-vascularized cornea and the lens.
• The Vitreous humor is A clear jelly-like substance filling the space between the lens and the retina
• It helps maintain a permanent fixed shape of eye
• The Sclera is a tough, white covering over the eye, except for the cornea
• It is protected by a transparent coating called the Conjunctiva
• The retina is the eyes light sensitive part
• The brain receives electrical nerve impulses, which are converted from light images.

Focusing Elements of the Eye

• The cornea bends or refracts light rays; amount depends on surface curvature and light speed.
• The index of refraction is nearly constant for all corneas; curvature variance lead to deflection.
• Indexes of refraction:
  • Cornea: 1.37
  • Aqueous humor: 1.33
  • Lens cover: 1.38
  • Lens center: 1.41
  • Vitreous humor: 1.33

Sensitivity of the Eye

• The retina is the light detector converting light images into electrical impulses sent to the brain.
• Light photon absorption in a photoreceptor initiates a photochemical reaction.
• This then initiates action potential, which produces electrical nerve impulses sent to the brain
• The macula lutea, or yellow spot, is where most vision is restricted.
• Detailed vision occurs in the fovea centralis, a tiny area (0.3mm diameter) within the yellow spot.
• Two photoreceptor types in the retina: cones and rods.
• Rods and cones are distributed symmetrically, with exception of the blind spot
• Blind spots contain nor rods or cones, covering 13° to 18° regions.
• Cones are mostly in the fovea centralis with 6.5 million in each eye, daylight vision & color recognition
• Cones all have individual nervous links to the brain and determine the amount of precise details one sees
• Cones have maximum sensitivity at about 550 nm in the yellow-green region.
• Rapid dark adaptation occurs with cones (5min).
• Rods cover most of the retina: 120 million in each eye, used for night and peripheral vision.
• Hundreds of rods share information to a same nerve fiber
• Ability of rods to resolve close light sources is poorer than cones
• Maximum sensitivity of rods occurs with blue-green light (~510nm).
• Rods will adapt to dark for 30 to 60 min.

Image Formation on the Retina

• Near objects need the eye muscles to contract
• This will shorten muscles and thicken+power the eye lens.
• Distant Objects eye muscles relax, muscle fibers legthen to flatten the eyes lens.

Near and Far Points

• The focal length is measured using the formula 1/f = 1/do + 1/di where di = 2 cm = 0.02 m.
• Near point: When 1/do = 0.25m will be 1/f = 4 + 50 = 54 m-1 . When looking through near point, the ciliary muscles will contract so that the focal point moves far away from the viewer
• Far point: When 1/d = 0 the formula becomes will be 1/f = 0 + 50 = 50 m-1. Relax ciliary muscle and it will move along the retina again

Accommodation

• Strength of the lens can thus by calculated via P in diopters and as 1/f(m).
• Convex convergence lenses have a positive lens power, while concave lenses have a negative power of lens.
• For normal vision:
  • At near point: P = (1/ 0.25) + (1/ 0.02) = 54D
  • At far point: P = (1/ ∞ ) + (1/ 0.02) = 50D
• Accommodation: difference 54-50 = 4D
• Combination of lenses that also can be calculated when its focal length is known via the formula of 1/F = 1/F1 + 1/F2 + ... + 1/Fn .
• When assuming lens A has a focal length of 0.33m and B is at 0.25m, the following combined focal point becomes: 1/F = (1/0.33) + (1/0.25) = 3 + 4 = 7 D.

Vision Defects

• Nearsightedness (Myopia)
• Near viewing is clear, but distant objects blur due to long eye ball.
• Result of having too strong of an eye lens.
• The eye over converges nearly parallel rays from a distant object.
• Correction by having or using a diverging eyeglass (concave) in front of eye to correct sight.
• Farsightedness (Hyperopia).
• distant viewing is clear, but distant are objects.
• Result of a weak eye lens or short eyeball.
• Insufficient convergence from a near object must meet on the retina
• Corrections achieved by using a convex lens over the eye.
• Presbyopia
• Results in the eyes inability to see objects.
• It is part of the natural aging process occurring around age 45.
• Bifocal lenses contain both concave and convex lenses
• The upper portion consists of a concave lens that facilitates distant viewing
• A lower convex lens allows for close up viewing as a near lens
• Stigmatism.
• When normal eye is present, point objects do not from a clear point and an unequal amount of eye curvature in different directions.
• Lens for correcting stigmata should be asymmetric.
• Correct any vision, one must calculate power of glass needed by finding formula P normal = Patient + P
• With Example/ Find the strength and the focal point lens that makes eye good with a 1m distance
• Pnormal = 1/f = 1/∞ + 1/0.02 -> norm = 0 + 50 = 50
• P normal = 1/fp = 1/1 +1/0.02 gives 1 +50=51 from patients POV
• P =PN + P from patients point gives 50-51 which results to -1 and is a concave lens
• This is a short example of using concave lens with a person having the same vision results
• HW is now needed for myopic male has near and far points of 20cm and 250 cm Spectacle and how correct the lens is need from defect
• HW will then make patient has a near point of 15cm without glasses
• And a correction lens needed for that person that is -1 which provides that far point.

Explore the anatomy of the eye, matching components to their functions in vision, light adaptation, and eye health. Delve into roles in detailed and peripheral vision and causes of vision problems.