The Eye (AQA GCSE Biology): Revision Note

Function of the eye

• The eye is a sense organ containing receptor cells which are sensitive to light intensity and colour

• The purpose of the eye is to receive light and focus it onto the retina at the back of the eye

• There are two main functions of the eye:

  • adaptation to bright or dim light

  • accommodation to focus on near or distant objects

Structure of the eye

The eye is an organ made from several different types of tissue; all of the structures function together to allow light to hit the retina, which sends signals to the brain

Adaptation and accommodation in the eye

Adaptation to dim and bright light

• The eye can adapt its structures in response to light intensity

• This adaptation is a reflex action carried out to protect the retina from damage in bright light and to protect us from not seeing objects in dim light

• The reflex action is controlled by muscles in the iris, which results in the following:

  • In dim light the pupil dilates (widens) to allow as much light into the eye as possible

  • In bright light the pupil constricts (narrows) to prevent too much light from entering the eye and damaging the retina

Bright light can damage the receptor cells of the retina; the pupil reflex protects the eye by altering the diameter of the pupil

Accommodation

• Accommodation is the process of changing the shape of the lens to focus on near or distant objects

• The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose

• Changing the shape of the lens alters how much light is refracted 

  • This is important in making sure that light is focused on the retina of the eye rather than in front or behind it

• The contraction or relaxation of the ciliary muscles brings about the changes

Focusing on a near object 

• The ciliary muscles contract

• The suspensory ligaments loosen

• The lens is then thicker and refracts light rays more strongly

Diagram showing the eye when an object is close up

Focusing on a distant object

• The ciliary muscles relax

• The suspensory ligaments are pulled tight

• The lens is then pulled thin and only slightly refracts light rays

Diagram showing the eye when an object is far away

Defects of the eye

• Two common defects of the eyes are

  • myopia (short-sightedness) 

  • hyperopia (long-sightedness)

• In both defects rays of light do not focus on the retina

• Generally, these defects are treated with spectacle lenses (glasses) which refract the light rays so that they focus on the retina

Treating eye defects

• New technologies are now available which can treat both of these defects rather than using spectacle lenses

• Treatments include:

  • Hard and soft contact lenses:

    • These sit on the surface of the eye and are almost invisible, making them ideal for activities like sports

    • Soft lenses are more comfortable but carry a higher infection risk

  • Laser surgery:

    • Lasers can be used to change the shape of the cornea (changing how it refracts light onto the retina)

    • All surgical procedures have a risk of unexpected damage occurring during the procedure which could lead to worse vision or an infection

    • For myopia: the cornea is slimmed down, reducing the refractive power

    • For hyperopia: the cornea shape is changed so the refractive power is increased

  • Lens replacement surgery

    • This surgery completely replaces the lens of the eye with a plastic artificial lens (rather than changing the shape of the cornea during laser eye surgery)

    • The procedure is more invasive than laser surgery and carries a risk of damage occurring to the retina leading to complete sight loss