Eye structure and function

• The eyeball sits in a socket of the skull called the orbit for protection.
• Three pairs of eye muscles attach the eyeball to the socket, enabling rotation.

Structures Protecting the Eye's Surface

• Eyebrows: Prevent sweat from entering the eye.
• Eyelashes: Trap dust and prevent it from entering
• Eyelids: Cover the eye to protect it from dirt and strong light
• Eyelids also spread tears to keep the eye moist.

Main Parts of the Eye and Their Functions

• Cornea:
  • Transparent layer allowing light to enter
  • Curved surface helps focus light onto the retina
• Iris: Controls the size of the pupil
• Pupil:
  • Opening at the iris center
  • Allows light to enter
• Lens:
  • Transparent, elastic, and biconvex
  • Focuses light onto the retina
• Ciliary Muscles: Change the lens thickness
• Jelly-like Fluid:
  • Maintains eyeball shape
  • Helps focus light onto the retina
• Sclera: Tough white coat that protects the eyeball
• Retina: Contains light-sensitive cells
• Optic Nerve: Transmits messages from the retina to the brain
• Blind Spot: Point on the retina where the optic nerve exits, lacking light-sensitive cells.

How We See

• Light enters the eye from an object.
• Light is bent and focused by the cornea, lens, and jelly-like fluid onto the retina, forming an inverted image.
• Light-sensitive cells send messages to the brain via the optic nerve.
• The brain interprets the messages as an upright image.

Light-Sensitive Cells on the Retina

• Rod Cells:
  • Vision in black and white
  • Sensitive to dim light
• Cone Cells:
  • Vision in colour
  • Sensitive to bright light

Lens Elasticity and Focus

• The lens is elastic, changing thickness to focus light from different distances.
• Near Objects: Lens becomes thicker to focus light onto the retina.
• Distant Objects: Lens becomes thinner to focus light onto the retina.

Vision Limitations

• Clear vision is not possible when objects are too near, too far, or too small.
• Instruments like hand lenses, microscopes, binoculars, and telescopes extend our vision.
• Objects cannot be seen when their image falls on the blind spot.

Short Sight vs. Long Sight

• Short Sight (Myopia):
  • Near objects are clear, distant objects are blurry
  • Caused by a lens that is too thick or an eyeball that is too long
  • Distant images are formed in front of the retina
  • Corrected with concave lenses
• Long Sight (Hyperopia):
  • Distant objects are clear, near objects are blurry
  • Caused by a lens that is too thin or an eyeball that is too short
  • Near objects form images behind the retina
  • Corrected with convex lenses

Eye Defects and Diseases

• Common eye issues include colour blindness (e.g., red-green), astigmatism, cataract, and glaucoma.