The Eye - IGCSE Biology Revision Notes

Overview

• The eye is a sense organ with receptor cells sensitive to light.
• Key structural features include:
  • Cornea
  • Iris
  • Lens
  • Retina
  • Optic nerve

Eye Structure and Functions

Pupil Reflex

• Pupil Reflex: Controls light entry by altering pupil diameter.
  • Dim light: Pupil dilates to let more light in.
  • Bright light: Pupil constricts to reduce light entry and protect the retina.
• Occurs due to changes in iris muscles:
  • Circular muscles: Form circles around the pupil.
  • Radial muscles: Radiate outward from the pupil.
  • Antagonistic muscles: One contracts while the other relaxes, regulating light entry.

Iris Muscles

Dim Light

• Radial muscles contract; circular muscles relax.
• Pupil dilates, allowing more light in.

Bright Light

• Radial muscles relax; circular muscles contract.
• Pupil constricts, reducing light entry.

Eye Accommodation

• Describes how the eye focuses on near or distant objects by changing the lens shape.
• Changes involve:
  • Contraction/relaxation of ciliary muscles.
  • Adjustment of tension in suspensory ligaments.

Focusing on Near Objects

• Ciliary muscles contract; suspensory ligaments loosen.
• Lens becomes more rounded; light is refracted more.

Focusing on Distant Objects

• Ciliary muscles relax; suspensory ligaments tighten.
• Lens becomes thinner; light is refracted less.

Rods & Cones

• Rods and cones are two types of receptor cells in the retina.
• Rods: Detect light at low levels; important for night vision.
• Cones: Detect light at different wavelengths; enable color vision.
• Distribution:
  • Rods: Found all over retina except for the blind spot.
  • Cones: Concentrated in the fovea for sharp, colored images.