The Eye (Cambridge (CIE) IGCSE Biology)

Eye structure

• The eye is a sense organ containing receptor cells that are sensitive to light

• The structure of the eye allows it to carry out its function; important structural features include the:

  • cornea

  • iris

  • lens

  • retina

  • optic nerve

Eye structure diagram

The structures of the eye allow it to detect light and transmit nerve impulses to the brain

Eye structure and function table

The pupil reflex

• The pupil reflex is an example of a reflex action; its role is to control the light that enters the eye by altering the pupil diameter

  • In dim light the pupil dilates in order to allow as much light into the eye as possible

  • In bright light the pupil constricts in order to prevent too much light entering the eye and damaging the retina

The pupil reflex causes the pupil to dilate in dim light and constrict in bright light

Iris muscles: extended

• The pupil reflex occurs due to changes in the iris muscles

  • The iris muscles work together to regulate the amount of light entering the eye

• The iris contains circular muscles and radial muscles

  • The circular muscles form circles around the pupil

  • The radial muscles radiate outwards from the pupil

• The circular and radial muscles of the iris are antagonistic, meaning that they work against each other

  • When one set of muscles contracts the other relaxes, and vice versa

Iris muscles in dim light

• When light levels are low the pupil reflex acts to dilate the pupil and maximise the light entering the eye; this is achieved as follows:

  • light receptors in the eye detect low light levels

  • the radial muscles contract and the circular muscles relax

  • the pupil dilates

Iris muscles in dim light diagram

In dim light the muscles of the iris work to dilate the pupil and maximise light entering the eye

Iris muscles in bright light

• When light levels are high the pupil reflex acts to constrict the pupil to reduce light entering the eye and protect the retina; this occurs as follows:

  • light receptors in the eye detect bright light

  • the radial muscles relax and the circular muscles contract

  • the pupil constricts

Iris muscles in bright light diagram

In bright light the muscles of the iris work to constrict the pupil and limit light entering the eye

The iris muscles and light levels summary table

Eye accommodation: extended

• Accommodation is the term used to describe the way in which the eye focuses on near or distant objects

• During eye accommodation the shape of the lens is changed, altering the extent to which light is refracted; this change is brought about by:

  • contraction or relaxation of the ciliary muscles

  • adjustment of tension in the suspensory ligaments

Eye accommodation and near objects

• When an object is close up:

  • the ciliary muscles contract

  • the suspensory ligaments loosen

  • the suspensory ligaments exert less pull on the lens, allowing the lens to become more rounded

  • light is refracted more

Eye accommodation and near objects diagram

When viewing a near object accommodation of the eye involves contraction of the ciliary muscles and loosening of the suspensory ligaments

Eye accommodation and distant objects

• When an object is far away:

  • the ciliary muscles relax

  • the suspensory ligaments tighten

  • the suspensory ligaments pull on the lens, causing it to become thinner

  • light is refracted less

Eye accommodation and distant objects diagram

When viewing a distant object accommodation of the eye involves relaxation of the ciliary muscles and tightening of the suspensory ligaments

Eye accommodation summary table

Rods & cones: extended

• Rods and cones are the two types of receptor cell present in the retina of the eye

• Rod cells and cone cells have different roles in detecting light stimuli: 

  • Rods can detect light at low levels, so play an important role in night vision

  • Three different types of cones can detect light at three different wavelengths, enabling colour vision

• Rods and cones are not distributed evenly across the retina:

  • Rod cells are found all over the retina, with the exception of the blind spot

  • Cone cells are concentrated in the fovea, the region of the eye onto which light is focused by the process of accommodation

    • The fovea enables the brain to form sharp, coloured images when light is effectively focused by the eye