Medical Physics: Vision and the Eye

Visual System

• The sense of vision consists of three major components: the eyes that focus an image from the outside world on the light-sensitive retina, the system of millions of nerves that carries the information deep into the brain, and the visual cortex where it is all put together.
• Blindness results if any one of these parts does not function.

Vision Elements of the Eye

• The cornea:
  • A clear, transparent bump on the front of the eye that does about two-thirds of the focusing of light.
  • Focuses by bending (refracting) light rays, with the amount of bending depending on the curvatures of its surfaces and the speed of light in the lens compared to the surrounding material.
• The iris:
  • The colored part of the front of the eye.
  • Aids the eye by increasing or decreasing incident light on the retina until it adapts to new lighting conditions.
  • Plays an important role in reducing lens defects under bright light conditions.
• The pupil:
  • A small opening in the center of the iris where light enters the lens.
  • Appears black because essentially all of the light that enters is absorbed inside the eye.
  • Can change from approximately 3mm in diameter in bright light to 8mm in diameter in dim light.
• The lens:
  • Variable in shape and has the ability to focus objects at various distances both at its front and back surfaces.
  • More curved in the back than in the front.
  • Focusing power is smaller than that of the cornea (1/3) because it is surrounded by substances that have indexes of refraction close to its own.

Focusing Elements of the Eye

• The cornea and lens are the two major focusing components of the eye.

Aqueous Humor

• Fills the space between the lens and the cornea.
• Mostly water (n ≈ 1.33).
• Continuously produced and surplus escapes through the drain tube ( Canal of Schlemm).
• Blockage of the drain tubes results in increased pressure in the eye (Glaucoma).
• Maintains the internal pressure of the eye at about 20mm Hg.
• Contains many of the components of blood and provides nutrients to the non-vascularized cornea and lens.

Vitreous Humor

• A clear, jelly-like substance that fills the large space between the lens and the retina.
• Helps to keep the shape of the eye fixed and is essentially permanent.

Sclera

• The tough, white, light-tight covering over all of the eye except the cornea.
• Protected by a transparent coating called the conjunctiva.

Retina

• The light-sensitive part of the eye that converts the light image into electrical nerve impulses that are sent to the brain.
• Most vision is restricted to a small area called the macula lutea, or yellow spot.
• All detailed vision takes place in a very small area in the yellow spot (0.3mm in diameter) called the fovea.

Eye Photoreceptors

• There are two general types of photoreceptors in the retina: the cones and the rods.
• Rods:
  • 120 million in each eye.
  • Used for night vision and peripheral vision.
  • Have a maximum density at an angle of about 20° from the visual axis.
• Cones:
  • 6.5 million in each eye.
  • Used for daylight, where we see fine details and recognize different colors.
  • Primarily found in the fovea.

Image Formation on the Retina

• The retina, the light-sensitive part of the eye, converts the light images into electrical nerve impulses that are sent to the brain.

Diopter Strength of the Eye

• The focal length F of a combination of two lenses with focal lengths F1 and F2 is given by:

Vision Defects

• Nearsightedness (Myopia):
  • Ability to see near objects clearly, whereas distant objects are blurry.
  • Due to the too strong eye lens or too long eyeball.
  • Corrected by placing a diverging eyeglass (concave) lens in front of the eye.
• Farsightedness (Hyperopia):
  • Ability to see far objects clearly, whereas near objects are blurry.
  • Due to the too weak eye lens or too short eyeball.
  • Corrected by placing a converging eyeglass (convex) lens in front of the eye.
• Presbyopia:
  • Loss of the eye's ability to change its focus to see objects that are near.
  • Part of the natural aging process of the eye.
  • Corrected by using bifocal lenses.