27. Human eye as an optical system

Optical Characteristics of the Eye

• The human eye functions as a complex optical system composed of the cornea, lens, aqueous and vitreous humors, and retina.

• Optical Power (P):
• Ranges 59–71 diopters (dpt) in total.
• Cornea contributes ~40 dpt (main refractive element).
• The crystalline lens contributes ~19–31 dpt, adjustable via accommodation.

Accommodation

• Definition: The eye’s ability to focus on objects at different distances.

• Achieved by changing the shape of the lens (not the pupil).
• Lens curvature increases → focus on near objects.
• Lens curvature decreases → focus on distant objects.
• Pupil also adjusts with light intensity:
• Constriction in bright light → reduces spherical aberrations.
• Dilation in dim light → increases light entry.
• Light energy is converted into electrical signals in the retina, transmitted via the optic nerve to the brain.

Aberrations of the Human Eye

1. 1. Spherical Aberration:

• Peripheral rays are focused in front of the retina compared to central rays.
• Causes blurred vision, especially in dim light.

1. 2. Chromatic Aberration:

• Different wavelengths (colors) refract differently → focus at different points.
• Produces color fringes (halos) around objects.

1. 3. Other Optical Aberrations:

• Can result from irregularities in cornea or lens shape, contributing to astigmatism.