CIE IGCSE Biology: 复习笔记:14.1.6 The Eye

CIE IGCSE Biology: 复习笔记:14.1.6 The Eye

Structure & Function of the Eye

 

 

  • 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 - Antagonistic Muscle Action: Extended

  • 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: Extended

 

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

Exam Tip

The focusing of the eye on distant and near objects is complex and it can be hard to remember what is happening. This is something you can work out in an exam if you have forgotten – staring at your hand right in front of your eye will make your eyes feel tight and tired after a few seconds. This is because the ciliary muscles are contracted. Staring at an object far away feels relaxing and comfortable because the ciliary muscles are relaxed.

Rod & Cone Cells: Extended

  • 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

Exam Tip

Do you ever wonder why your night vision is black and white? It's because the low light intensity isn't enough to stimulate cone cells, so only rod cells are stimulated. Remember, only the cone cells can detect colour.

Also, if you look directly at a dim star at night, it disappears, but reappears if you look slightly to one side of it. This is because, when looking straight at the star, the light falls on the fovea, which has more cones so fewer rods, so the low light intensity won't be enough to stimulate the rods. Looking to the side allows the light to fall away from the fovea, onto more rod cells, so the start reappears!

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