The lens is made up of protein fibers arranged in a specialized way that results in transparency.  The lens has four layers: At the center is the nucleus, which is surrounded by the next layer, the cortex. Surrounding the cortex is the lens epithelium. The layer at the surface is the lens capsule.

In healthy eyes, light rays reflected from an object enter the eye through the cornea and lens, which together focus the light onto the retina to produce a sharp image. When a cataract develops, however, light rays are no longer precisely focused.  Instead, the rays are scattered before reaching the retina.

The three common types of cataracts are defined by where they occur in the lens: nuclear, cortical and posterior subcapsular (in the rear of the lens capsule). It is possible for a person to have more than one type of cataract in the same eye.

Nuclear cataracts:

  • Most common type
  • Chances increase with age and smoking

Cortical cataracts:

  • Become more common with age
  • Related to lifetime exposure to ultraviolet light

Posterior subcapsular cataracts:

  • Most likely to occur in younger people
  • Result of prolonged use of steroids, inflammation trauma or diabetes

Source: The Johns Hopkins White Papers