Structure of the Eye

The process of vision cannot be understood without some knowledge about the structure of the eye:

  • The cornea is the transparent, protective outer membrane of the eye.
  • The iris, the colored part of the eye, is a ring of muscle.
  • The iris surrounds an opening called the pupil, which can get bigger or smaller to allow different amounts of light through the lens to the back of the eye. In bright light, the pupil contracts to restrict light intake; in dim light, the pupil expands to increase light intake.
  • The lens, which lies behind the pupil and iris, can adjust its shape to focus light from objects that are near or far away. This process is called accommodation.
  • Light passing through the cornea, pupil, and lens falls onto the retina at the back of the eye. The retina is a thin layer of neural tissue. The image that falls on the retina is always upside down.
  • The center of the retina, the fovea, is where vision is sharpest. This explains why people look directly at an object they want to inspect. This causes the image to fall onto the fovea, where vision is clearest.

Rods and Cones

The retina has millions of photoreceptors called rods and cones. Photoreceptors are specialized cells that respond to light stimuli. There are many more rods than cones. The long, narrow cells, called rods, are highly sensitive to light and allow vision even in dim conditions. There are no rods in the fovea, which is why vision becomes hazy in dim light. However, the area just outside the fovea contains many rods, and these allow peripheral vision.

Because rods are so sensitive to light, in dim lighting conditions peripheral vision is sharper than direct vision.

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