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The Eye

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The Structure of the Eye

Click here for a diagram of the eye.

Parts of the eye and their function:
Conjunctiva
This is a transparent membrane that covers and protects the front of the eye.

Sclera
This is the white part of the eye. It does not allow light to pass through. Its function is protection and shape.

Cornea
This is the transparent part of the sclera at the front of the eye. It can bend the light inward because of its convex shape.

Choroid
This layer is darkly pigmented to prevent light reflecting within the eye. It has blood vessels which nourish the eye.

Retina
It has the light receptors rods and cones.
Rods
  • Located towards the sides of the retina
  • Sensitive to dim light
  • Cannot determine colour
  • The pigment in the rods is called Rhodopsin.
Cones
  • Located at the fovea (yellow spot)
  • Sensitive to bright light
  • Can determine colour
  • There are three different cone pigments
Iris
This is the coloured part of the eye. It controls the size of the pupil, therefore controls the amount of light entering the eye.

Pupil
A hole in the iris. It allows light into the eye.

Lens
A biconvex transparent elastic structure. It focuses light onto the retina.

Ciliary body
Contains muscle that alters the shape of the lens.

Suspensory ligaments
Fibres that connect the ciliary body to the lens.

Vitreous humour
A clear jelly like substance. It supports the eye by exerting an outward pressure.

Aqueous humour
A clear liquid that helps maintain the shape of the front of the eye.

Optic nerve
Carries nerve impulses from the retina to the brain.

Blind spot
Part of the retina where the optic nerve leaves the eye. There are no rods or cones at this point.

Fovea
The area of the retina that has most accurate vision.


Accomodation

This is the ability of the lens to change its curvature to focus near or distant objects.

Distant object
(a) Ciliary muscle relaxes
(b) The suspensory ligaments are pulled tight
(c) The lens is stretched into a flatter shape.

Near object
(a) Ciliary muscle contracts
(b) The suspensory ligaments are no longer under tension
(c) The lens become thicker and more convex.

Click here for diagrams illustrating accomodation

Light Intensity

How the eye adjusts to changing light intensity
1. Dim light
  • The radial muscles of the iris contract.
  • The circular muscles of the iris relax.
  • The pupil enlarges to increase the amount of light entering the eye.
2. Bright light
  • The radial muscles of the iris relax.
  • The circular muscles of the iris contract.
  • The pupil gets smaller to decrease the amount of light entering the eye.
Click here for diagrams illustrating eye adjusting to light intensity.

Binocular Vision

In humans the two eyes are positioned so that an object can be viewed by both eyes at the same time. Each eye views the object from a slightly different angle, thus producing different images. The brain combines these images giving the perception that the object is three dimensional.

Eye defects

Shortsightedness or Myopia
  • Can see near objects but not distant objects.
  • This is where the eye ball is too long from front to back.
  • The light from a distant object is brought to a focus in front of the retina.
  • Can be corrected by using glasses (or contact lenses) with concave (diverging) lens.
Longsightedness or Hypermetropia
  • Can see distant objects but not near objects.
  • This is where the eye ball is too short from front to back.
  • The light from a near object is brought to a focus behind the retina.
  • Can be corrected by using glasses (or contact lenses) with convex (converging) lens.
Click here for diagrams illustrating short sight and long sight.

Experiments

Experiment 1
To demonstrate binocular vision

Experiment 2
To find the blind spot


LC higher level exam questions

1993 Q. 15 (b)
In each of the following cases explain how the event indicated is brought about.
(i) The control of light entering the eye.
(ii) The focussing of light rays on to the retina when a person commences reading.
(iii) The detection, by binocular vision, that an object is three dimensional.
Solution

1997 Q.15 (d) (vii)
Give a biological explanation for the following:
Colour vision deteriorates as daylight declines in the evening.
Answer:
Colour is detected by the cones. The cones function only in brighter light.

1999 Q.12 (f)
Give a concise biological explanation for the following:
As the evening progresses and it gets darker, a time is reached when the human eye can no longer detect colour.
Answer:
Colour is detected by the cones. The cones function in bright light but cease to function in dim conditions.

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