The Eye

👁 The Eye – AQA GCSE Biology (Triple Science)

Definition

  • The eye = a sense organ containing receptors sensitive to light intensity and colour.

Main Structures & Functions

  • Cornea – transparent front of eye; refracts (bends) light, begins focusing.

  • Iris – coloured part; controls pupil size (amount of light entering).

  • Pupil – hole in centre of iris; allows light to pass through.

  • Lens – changes shape to finely focus light on retina (process = accommodation).

  • Retina – layer at back of eye; contains receptor cells:

    • Rods = detect light intensity (black/white).

    • Cones = detect colour.

  • Optic nerve – carries electrical impulses from retina to brain.

  • Sclera – tough white outer layer; protects the eye.

  • Ciliary muscles + suspensory ligaments – work with lens for focusing (distant vs near objects – explained in accommodation video).

Pupil Reflex (Homeostasis)

  • Dim light

    • Light receptors in retina detect low light.

    • Impulses → brain → iris muscles.

    • Radial muscles contract → pupil dilates (widens).

    • Allows more light into eye.

  • Bright light

    • Light receptors detect high light intensity.

    • Impulses → brain → iris muscles.

    • Circular muscles contract → pupil constricts (narrows).

    • Protects retina from damage.

  • Reflex action = automatic (does not involve conscious brain).

✅ Key takeaways:

  • Know structures and functions (cornea, iris, pupil, lens, retina, sclera, ciliary muscles, suspensory ligaments, optic nerve).

  • Reflex control of pupil size protects retina.

  • Triple-only: detail on pupil reflex mechanism (dim vs bright light).