Physics of Eyes and Vision

Physics of Eyes and Vision

• The sense of vision involves three major components: the eyes, a nervous system, and the visual cortex in the brain
• The eyes focus images onto the light-sensitive retina
• The nervous system carries the information to the brain
• The visual cortex interprets the information resulting in vision
• Blindness can result from any part not functioning properly

Topics of the Lecture

• Visual System
• Vision Elements of the Eye
• Focusing elements of the eye
• Sensitivity of the eye
• Image formation on the retina
• Vision Defects

Vision Elements of the Eye

• Cornea: The clear, transparent front part of the eye; responsible for about two-thirds of focusing.
• Eye's focusing components:
  • Cornea: A fixed focus element
  • Lens: A variable focus element; adjusts shape to focus objects at varying distances
• Iris: The coloured part of the eye; controls the size of the pupil to regulate light entering the eye
• Pupil: The opening in the center of the iris; controls the amount of light entering the eye
• Lens:
  • Focusing power is less than the cornea, as it's surrounded by fluids with similar refractive indices.
  • Made of numerous layers, similar to an onion.
• Aqueous humor: Fills the space between the lens and the cornea; mostly water. Maintains internal pressure at 20mm Hg
  • Produced continually, surplus drains via Canal of Schlemm. Blockage leads to increased pressure (Glaucoma)
  • Provides nutrients to the non-vascularized cornea and lens.
• Vitreous humor: A clear, jelly-like substance that fills the large space behind the lens, helping to maintain the eye's shape.
• Sclera: The tough, white outer layer of the eye, that protects the internal structures except the cornea and is protected by the conjunctiva.
• Retina: The light-sensitive layer of the eye; converts light images into electrical nerve impulses sent to the brain
• Photoreceptors:
  • Rods and cones: Responsible for converting light into electrical signals
  • Cones: Concentrated in fovea centralis (center of retina); for daylight vision, color perception.
  • Rods: Throughout the retina, responsible for night vision, low-light adaptation
  • Blind spot: Where the optic nerve leaves the retina; lacks photoreceptors
• Macula lutea or yellow spot: Area in the retina where detailed vision occurs. Located in the fovea centralis with a very small, well-defined region of high-density photoreceptors.

Focusing Elements of the Eye

• The cornea refracts light, bending it to focus on the retina. The degree of bending is determined by the cornea's surface curvature and the speed of light.
• The index of refraction is nearly consistent for all corneas, but the curvature varies among individuals, which contributes to refractive errors (common vision defects)

Sensitivity of the eye and Image Formation on the Retina

• Retina converts light images into electrical signals sent to the brain
  • Light hitting photoreceptors causes a chemical reaction to initiate electrical impulses.
• Detailed vision takes place in the macula lutea, a region in the retina which contains the fovea centralis.

Vision Defects

• Nearsightedness (Myopia): Distant objects are blurry, the eye lens is too powerful or eyeball too long. Corrected by a diverging lens (concave)
• Farsightedness (Hyperopia): Near objects are blurry, eye lens is too weak or eyeball too short. Corrected by a converging lens (convex)
• Presbyopia: Gradual loss of the eye's ability to focus on near objects with age. Corrected by bifocal lenses – Combining a converging lens for close-up vision with a diverging lens for distance vision
• Astigmatism: Unequal curvature of the cornea or lens, causing blurry vision at all distances. Corrected by astigmatism-correcting lenses.

How Does Vision Work?

• The eye focuses light onto the retina for the formation of an inverted image
• The muscles adjust the lens thickness to focus on near or far objects
• Near objects cause the eye muscles to tense up to increase the lens thickness and focus the light rays.
• Far objects need less lens thickness

Image Formation on the Retina

• The eye's muscles control the shape of the lens to focus on objects at various distances.
• For near objects, the ciliary muscles contract and the lens becomes thicker, increasing its refractive power.
• For distant objects, the ciliary muscles relax and the lens becomes thinner, decreasing its refractive power.

Near and Far Points

• Near point: The closest point an object can be brought into focus by the eye's lens
• Far point: The farthest point an object can be brought into focus by the eye's lens
  • A normal person has a near point of 25 cm and far point of infinity

Accommodation

• The ability of the eye to change the focal length of the lens to focus on objects at different distances
• Expressed using optical power (P), where (1/f) = P (diopter)

Diopter Strength of the Eye

• Diopter strength is a measurement of the optical power of a lens
• The strength of a combination of lenses is the sum of the individual powers

Homework Problems

• Presented in the lecture

Explore the fascinating components of the visual system in this quiz. Learn about the eyes, their focusing mechanisms, the nervous system's role, and how the visual cortex interprets what we see. Test your understanding of vision elements and related defects.