The Eye (OCR GCSE Biology A (Gateway))

Revision Note

Ruth Brindle

Last updated

The Human Eye

  • 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

The eye is a sensory organ made up of several different tissue types

Eye structure & function table

Function of the parts of the eye table, IGCSE & GCSE Biology revision notes

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 bright light

The pupil reflex in bright light

The pupil reflex in dim light

The pupil reflex in dim light

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

Diagram showing the eye when an object is close up

Diagram showing the eye when an object is close up

Eye when an object is far away

Diagram showing the eye when an object is far away

  • 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

Table summarising the changes in the eye when focusing on close or distant objects

The eye when an object is far away table

Common Eye Defects

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

Short sightedness 1

Short-sightedness occurs when light is refracted too much

Treatment of short-sightedness

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

Short sightedness 2

Short-sightedness is treated 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

Long sightedness 1

Long-sightedness occurs when light is not refracted enough

Treatment of long-sightedness

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

Long sightedness 2

Long-sightedness is treated using 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

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Ruth Brindle

Author: Ruth Brindle

Expertise: Biology

Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. With 10 years of teaching experience across the 3 key science disciplines, Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.