The Human Eye and Color Vision

The Human Eye and the Colourful World

• The human eye is a sensory organ that allows us to see colors and form images on the retina.
• It functions similarly to a camera, projecting an image onto a light-sensitive screen.
• Key components include:
  • Cornea: Transparent bulge allowing light entry; primary refraction occurs here.
  • Iris: Dark muscular diaphragm that controls pupil size.
  • Pupil: Regulates the amount of light entering the eye.
  • Retina: Delicate membrane containing light-sensitive cells; generates electrical signals sent to the brain via the optic nerve.
  • Crystalline Lens: Adjusts focal length for focusing objects.

Power of Accommodation

• The eye's lens adjusts its focal length to focus on objects at various distances, controlled by ciliary muscles.
• Distant objects: Ciliary muscles relax, making the lens thinner, increasing focal length.
• Nearby objects: Ciliary muscles contract, making the lens thicker, decreasing focal length.
• Least Distance of Distinct Vision (Near Point): Minimum distance for clear vision without strain—25 cm for normal eyes.
• Far Point: Maximum distance for clear vision—infinity for normal eyes.

Cataract

• Cataract: Cloudiness of the lens, often age-related.
• Partial or complete vision loss may result.
• Cataract surgery can restore vision.

Notes on Defects of Vision and Their Correction

• Refractive defects are common vision impairments.
  • Myopia (Nearsightedness): Distant objects are blurry, but nearby objects are clear.
    • Excessive curvature of the eye lens or elongation of the eyeball.
    • Image forms in front of the retina.
    • Corrected with concave lenses.
  • Hypermetropia (Farsightedness): Nearby objects are blurry, but distant objects appear clear.
    • Lens focal length too long or eyeball too small.
    • Image forms behind the retina.
    • Corrected with convex lenses.
  • Presbyopia: Difficulty focusing on nearby objects due to aging.
    • Ciliary muscles weaken and lens loses flexibility.
    • Corrected with bifocal lenses.

Refraction and Dispersion of Light

• A prism has two triangular bases and three rectangular sides.
• Refraction: Light bends as it enters a denser medium (e.g., glass).
• Dispersion: White light separates into its component colors (VIBGYOR) when passing through a prism.
  • Red light bends the least, violet light bends the most.
• Rainbows: Created by the dispersion and reflection of sunlight in water droplets.

Atmospheric Refraction and Scattering of Light

• Atmospheric refraction: Bending of light by Earth's atmosphere due to varying air density.

  • Twinkling of stars: Starlight bends and fluctuates due to atmospheric instability.
  • Planets do not twinkle: Light from planets, being from many points simultaneously, averages out.
  • Advance sunrise and delayed sunset: Apparent flattening and positioning of the sun are due to atmospheric refraction.

• Scattering of light: Light scattering by the atmosphere can cause blue skies.

  • Tyndall effect: scattering of light by colloidal particles.
  • Colour of the sky: Fine particles in the atmosphere scatter shorter wavelengths (blue) more than longer wavelengths (red).
  • Red signals: Less scattering, resulting in readability over longer distances.

Eye Donation

• After death, eyes can be donated for restoring eyesight in people with corneal blindness.
• Eyes can come from any age, gender, with or without conditions like glasses, or cataracts.
• Process quick and painless, approximately 10-15 minutes, for a donor of any age.

Key Definitions

• Accommodation: The eye's ability to adjust its focal length for clear vision at various distances.
• Near Point: The closest distance for clear vision without straining.
• Far Point: The farthest distance for clear vision.