Eyeballs

Parts of the eye

• Sclera - The white of the eye
• Cornea is the clear outer layer of the eye through which you can see the Iris and Pupil. It has an Index of Refraction of 1.376, and a curved outer surface, contributing to the refractive state of the eye. The Cornea provides about 80% of the eye's total refracting power. If you have LASIK or PRK surgery it thins the cornea to change your refractive state.
• Aqueous humor - the fluid supporting the cornea
• Pupil - the hole where light enters the eye
• Iris - the Iris is the colored part of the eyeball that contains the muscles that control the opening size of the pupil.
• Ciliary muscle is a ring of muscle fibers in the eye that control the tendons supporting the natural lens of the eye, and controls the flow of aqueous humor behind the cornea. The Ciliary muscle is controlled by the Ciliary ganglion, which is a complex intersection of several nerve systems. The action of the ciliary muscle is the primary source of accommodation and ciliary spasm which causes pseudomyopia.
• Lens - The part that changes the focus distance of the eye
• Rods and cones - Rods and Cones are the sensory cells in the back of your eye that detect light.
  • Rods sense only light intensity, not color. They sense edges more sharply than cones. If you are outside at dusk, you may feel a sudden switch of your vision from color vision to black and white, this is your visual cortex switching to only rod input when cone input isn't working as well in dim light.
  • Cones are the cells that detect color in your eye. There are three different types of cones that respond most strongly to three different wavelengths of light. Your visual cortex takes the combined response of the three types of cones and makes up the blended color in your mind. Magenta for example is an imaginary color. It's the color your brain makes up to explain why both short and long wavelengths of light are detected, but not the wavelengths in the middle. Most colors are on the color spectrum you learned in school (Red, Orange, Yellow, Green, Blue, Indigo, Violet), and will trigger a single cone type, or two adjacent cone types.

• Retina - the tissue that supports the rods and cones.
• Vitreous humor is the clear gel filling the majority of the eyeball. It is where true floaters live. This gel is important for helping the eye hold its shape and maintain the correct pressures inside the eye even when air pressure changes. In adults, the gel has a complex structure, with different thicknesses in different parts.
• Vitreous detachment is attached to the outer wall of the eye in multiple locations, but can become separated. When separated from the retina it does not support the retina fully, and puts you at higher risk for retinal detachment. It can also leave behind a large floater that impairs vision.

Focusing Power

As a back-of-the-envelope calculation for the focusing power of an emmetropic eye, we might estimate the axial length as around 2.5cm. For distance vision (parallel incident light) that number is simply the focal length of the eye at rest, giving 40 Diopters. If we take the near point as about 25cm, that requires an additional 4 dpt of focusing power from the lens. (For example using the thin lens equation ${\frac {1}{x}}+{\frac {1}{y}}={\frac {1}{f}}$.)

Axial Length

The primary cause of differences in refractive state is the length of the eye, referred to as axial length, relative to the focusing power. Long eyeballs are associated with myopia, as the natural lens of the eye, even when fully relaxed, focuses light too far forward of the retina.

See Also

• Duochrome Test