Eye Anatomy and Light Refraction

The Cornea and Lens Focus Light on the Retina

Visible Light

• Light consists of packets of energy called photons that travel in a wavelike fashion at high speeds.
• Visible light can be broken down into bands of colors (rainbow).
• The color perceived by the eye is a reflection of a particular wavelength.
  • Example: Grass appears green because it reflects green light and absorbs other colors.
  • White reflects all colors, while black absorbs all colors.

Light Path

• Light travels in straight lines and is blocked by non-transparent objects.
• The speed of light changes when it passes from one transparent medium to another with a different density.

Light Refraction

• Refraction: The bending of light rays.
• Occurs due to the change in the speed of light when it passes from one transparent medium to another at an oblique angle.

Lenses and Refraction

• Lenses in the eyes can refract light because they are curved.
• Convex lenses: Thicker in the center than at the edges.
• Concave lenses: Thicker at the edges than in the center.

How Lenses Refract Light

• Light hitting the curve of a lens at an angle is refracted (bent).
• Convex lenses: Bend light passing through them, causing rays to converge at a single point called a focal point.
• The more convex the lens, the more the light bends, and the shorter the focal distance.
  • Focal distance: The distance between the lens and the focal point.
• Concave lenses: Disperse light.

Image Formation

• The image formed at the focal point is upside-down and reversed from left to right.
• The brain corrects this.

Focusing Light onto the Retina

• Pathway of light entering the eye:
  • Cornea → aqueous humor → lens → vitreous humor → entire neural layer of the retina → photoreceptors
• Light is refracted three times along this path:
  • Entering the cornea.
  • Entering the lens.
  • Leaving the lens.

Refractory Power

• The majority of the refractory power is in the cornea.
• However, the cornea's focus is constant and cannot change.
• The lens can adjust its curvature to allow for fine focusing, enabling focus for both distant and close vision.

Focusing for Distant Vision

• Eyes are best adapted for distant vision.
• Emmetropia (normal eye state): A state of vision where a faraway object at infinity is in sharp focus with the eye lens in a neutral or relaxed state.
• Ciliary muscles are completely relaxed in distance vision.
  • This causes a pull on the ciliary zonule (suspensory ligaments).
  • Result: The lenses are stretched flat.

Focusing for Close Vision

• Light from close objects diverges as it approaches the eye.
• Focusing for close vision requires the eye to make active adjustments using three simultaneous processes:
  • Accommodation of the lens: Changing lens shape to increase refraction (focusing).
    • Ciliary muscle contracts → loosens ciliary zonule → lens bulges.

Adjustments for Close Vision

• Focusing for close vision requires eye to make active adjustments using three simultaneous processes:
  • Constriction of the pupils
    • Accommodation pupillary reflex: Constriction of pupils to prevent most divergent light rays from entering the eye.
    • The sphincter pupillae muscle accomplishes this.
  • Convergence of the eyeballs
    • Medial rotation of eyeballs causes convergence of eyes toward the object being viewed.
    • The medial rectus muscles accomplish this.

Problems with Refraction

• Emmotropic eye: normal eye.
• Myopia (nearsightedness):
  • The eyeball is too long, so the focal point is in front of the retina.
  • Corrected with a concave lens, which diverges the light before it enters the eye.

More Refraction Problems

• Hyperopia (farsightedness):
  • The eyeball is too short, so the focal point is behind the retina.
  • Corrected with a convex lens, which helps converge the light more strongly.

Astigmatism

• Unequal curvatures in different parts of the cornea or lens.
• Corrected with cylindrically ground lenses or laser procedures.

Presbyopia

• Loss of accommodation over the age of 50.