the eye

bright light

• pupil constricts

• circular muscles contract

• radial muscles relax

dim light

• pupil dilates

• circular muscles relax

• radial muscles contract

retina

contains light sensitive cells

• cones detect colour, but require high light intensities

• rods only detect black and white, but function in lower light intensities

optic nerve

nerve composed of many sensory neurones carrying impulses to the brain

scelera

tough white protective layer

cornea

refracts light through pupil

iris

controls aperture of pupil

ciliary muscles

control the thickness of the lens to allow focusing

suspensory ligaments

attach the ciliary muscles to the lens

fovea

where the majority of the light is focused and almost entirely consisting of cone cells

pupil

hole through which light enters the eye

two common defects of the eye

myopia (short sightedness) and hyperopia (long sightedness) in which rays of light do not focus on the retina

how are these defects generally treated ?

with spectacle lenses which refract the light rays so that they do focus on the retina

distant object

• the ciliary muscles relax

• the suspensory ligaments are pulled tight

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

near object

• the ciliary muscles contract

• the suspensory ligaments loosen

• the lens is then thicker and refracts light rays strongly

accommodation

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

new technologies to treat defects of the eye ?

hard and soft contact lenses, laser surgery to change the shape of the cornea and a replacement lens in the eye