The Eye (OCR Gateway GCSE Biology)

• The eye is a sense organ containing receptors sensitive to light intensity and colour
• Receptors are groups of specialised cells that can generate an electrical impulse in a sensory neurone
• The eye contains two types of receptor cell: rod cells which are sensitive to light intensity and cone cells which are sensitive to different wavelengths of visible light (colour)

The structure of the eye

• The purpose the eye is to receive light and focus it onto the retina at the back of the eye
• The retina is where the rod and cone cells are located

Eye structure & function table

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

Focussing 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

• To focus on a close up object:
  • Ciliary muscles contract
  • Suspensory ligaments slacken
  • This allows lens to become fatter
  • Light is refracted more

• To focus on a distant object:
  • Ciliary muscles relax
  • Suspensory ligaments are pulled tight
  • The lens is pulled thinner
  • Light is refracted less

Short-sightedness

• Short-sightedness is also called myopia
• It happens when the lens is more curved than normal or the eyeball is too long which means the light is refracted too much and so the Focal Point falls in front of the retina (rather than on the retina)
  • This means that distant objects appear blurry

Treatment of short-sightedness

• Short-sightedness can be corrected using contact lenses or glasses with a concave lens

Long-sightedness

• Individuals who suffer from long-sightedness can see distant objects clearly
• It happens when the lens is less curved than normal or the eyeball is too short which means the light is not refracted enough and so the focal point falls behind the retina (rather than on the retina)
  • This means that close objects appear blurry

Treatment of long-sightedness

• Long-sightedness can be corrected using contact lenses or glasses with a convex lens

Colour blindness

• People who suffer from colour blindness cannot distinguish between certain colours and in rare cases, cannot see colours at all
  • This happens because the cones in the retina do not work properly or are absent

• It is a genetically inherited condition but can also develop over time
• There are several different types of colour blindness, the most common is red-green colour blindness

Treatment of colour-blindness

• There is currently no cure for colour blindness (as the cone cells cannot be replaced) so most sufferers learn to live with the condition
• Some sufferers choose to use tinted lenses to emphasize some colours more