The Eye (CIE IGCSE Biology)

• The eye is a sense organ containing receptor cells that are sensitive to light (rod cells) and colour (cone cells)

The eye

 Function of the parts of the eye:

The Blind Spot

• At the point where the optic nerve joins the retina, there are no light-sensitive rod and cone cells on that part of the retina
• Light falling onto that part of the retina will not result in an image being detected
  • the brain 'fills in' from surrounding light so we don't see a black hole where no light has fallen
• This causes a blind spot, where we cannot detect an object in our peripheral vision even if it is there

The Pupil Reflex

• This is a reflex action carried out to protect the retina from damage in bright light and protect us from not seeing objects in dim light
• In dim light the pupil dilates (widens) in order to allow as much light into the eye as possible
• In bright light the pupil constricts (narrows) in order to prevent too much light entering the eye and damaging the retina

The pupil reflex

• In dim light, the pupil dilates (becomes larger) to allow more light to enter the eye to improve vision. 
• In bright light, the pupil constricts (gets smaller) to allow less light to enter the eye to protect the retina from damage.

• The pupil reflex is an example of a pair of antagonistic muscle groups acting together
• They work together to regulate the amount of light entering the eye
• The muscles that work antagonistically are the radial muscles and the circular muscles of the eye
  • When one set of muscles contracts, the other relaxes

The pupil reflex in dim light

The pupil reflex in bright light

Responding to Changes in Light Intensity Table

Accommodation: The function of the eye in focusing on near and distant objects

• The way the lens brings about fine focusing is called accommodation
• The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose
• The changes are brought about by the contraction or relaxation of the ciliary muscles
• When an object is close up:
  • The ciliary muscles contract (the ring of muscle decreases in diameter)
  • This causes the suspensory ligaments to loosen
  • This stops the suspensory ligaments from pulling on the lens, which allows the lens to become fatter
  • Light is refracted more

Diagram showing the eye when an object is close up

• When an object is far away:
  • The ciliary muscles relax (the ring of muscle increases in diameter)
  • This causes the suspensory ligaments to tighten
  • The suspensory ligaments pull on the lens, causing it to become thinner
  • Light is refracted less

Diagram showing the eye when an object is far away

Focussing on Distant and Near Objects Table

• There are two types of receptor cells in the retina:
  • Rods, which are sensitive to dim light
  • Cones, which distinguish between different colours in bright light
• There are 3 types of cone cells which are sensitive to different colours of light (red, blue and green)
• The fovea is an area on the retina where almost all of the cone cells are found
• Rod cells are found all over the retina, other than the area where the optic nerve attaches to the retina - there are no light-sensitive cells at all in this area, and so it is known as the blind spot