The Human Eye

STRUCTURE AND ROLES

Sclera: the tough and fibrous outer layer of the eye.

Choroid layer: the middle layer of the eye.  It contains blood vessels and melanin pigment.

Retina: the inner layer of the eye.  It contains light-sensitive rod and cone cells.

Cornea: the front, transparent part of the sclera that refracts light rays entering the eye.

Conjunctiva: a membrane surrounding the cornea, lubricated by watery tear fluid.

Iris: a modification of the choroid layer at the front of the eye.  It contains smooth radial muscle and circular muscle.  Contraction and relaxation of the iris muscles change the change the size of the pupil and allow variable amounts of light to enter the eye.

Pupil: a hole in the iris that becomes wider in dim light.

Optic nerve: conveys nerve signals to the brain.  (The light-sensitive rod and cone cells in the retina connect to sensory neurones that leave the retina at the blind spot and form the optic nerve).

Lens: a transparent disc of tissue that changes shape for near or far vision.

Suspensory ligaments: holds lens within the ring of ciliary muscle.

Ciliary muscle: a ring of muscle that alters the shape of the lens.

Aqueous humour: a clear, watery fluid in front of the lens.

Vitreous humour: a clear, jelly-like fluid behind the lens.

FOCUSING LIGHT

When light rays enter the eye they are refracted (bent).  First, the light is refracted by the cornea at the front of the eye.  Most of the refraction occurs here because it is curved.  The light is then refracted further by the lens, which focuses (converges) the light so that it forms a clear image on the retina.

There are three refracting surfaces: the cornea, the front surface of the lens, and the rear surface of the lens.

ACCOMMODATION

• Definition: the flattening or thickening of the lens by the ciliary muscles depending on the distance of an object from the eye.

Accommodation causes light to be refracted to a greater or lesser extent depending on how far the object is from the eye.

DISTANT OBJECTS

Light rays from a distant object are …