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

Use this image

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; it protects the retina from damage in bright light and makes objects visible in dim light

• The reflex action is controlled by muscles in the iris which affects pupil size:

  • In dim light the pupil dilates (widens) to allow more light into the eye

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

Use this image

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

Use this image

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

Use this image

Table outlining accommodation in the eye

Defects of the eye

• Two common defects of the eye:

  • 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

Use this image

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