The human eye
Objectives
relate the structure of the human eye to its functions as a sense organ
explain accommodation; sight defects and the corrections of each
The human eye
The eye detects light that has been reflected from an object and converts it
into nerve impulses. The impulses are transmitted along the optic nerve to the
brain which translates them into a precise picture of the object. The eyes are
situated in bony sockets of the skull called orbits and have muscles attached
to move them. The orbits protect the back of each eye from damage, and the
eyelids and eyelashes protect the front from foreign particles. Tears, produced
by tear glands above each eye, keep the eyes moist, wash away foreign
particles and contain an enzyme that destroys micro-organisms
Image formation
In order to see, light rays from an object must be refracted (bent) as they
enter the eye so that they form a clear image of the object on the receptor
cells of the retina. Being convex in shape, both the cornea and the lens refract
the light rays
Detection of light intensity and colour by the eye
The retina is composed of two types of specialised light-sensitive cells or
photoreceptors:
• Rods function in low light intensities. They are responsible for detecting the
brightness of light and are located around the sides of the retina. Images
falling on the rods are seen in shades of black and white only.
• Cones function in high light intensities. They are responsible for detecting
colour and fine detail, and are mainly located around the back of the retina.
The fovea is composed entirely of cones which are packed closely together.
There are three types that detect either the red, green or blue wavelengths of
light
Focusing light onto the retina – accommodation
By changing shape, the lens makes fine adjustments to focus the light rays
onto the retina. Changing the shape of the lens to focus light coming from
different distances onto the retina is called accommodation and it is brought
about by the ciliary muscles.
Control of light entering the eye
The size of the pupil controls the amount of light entering the eye. Muscles of
the iris control the pupil size.
Sight defects and how they are corrected
Short sight (myopia)
A person with short sight can see near objects, but distant objects are out of
focus. Light rays from near objects focus on the retina; light rays from distant
objects focus in front of the retina. Short sight occurs if the eyeball is too long
from front to back or if the lens is too curved (thick). It is corrected by wearing
diverging (concave) lenses as spectacles or contact lenses
Long sight (hypermetropia)
A person with long sight can see distant objects, but near objects are out of
focus. Light rays from distant objects focus on the retina; light rays from near
objects focus behind the retina. Long sight occurs if the eyeball is too short
from front to back or if the lens is too flat (thin). It is corrected by wearing
converging (convex) lenses as spectacles or contact lenses.
Glaucoma
Glaucoma is a condition in which the pressure of the fluid within the eye
increases due to the flow of aqueous humour from the eye being blocked. If
left untreated, the optic nerve becomes damaged and it can lead to blindness.
The most common type develops slowly and causes a gradual loss of
peripheral (side) vision. Glaucoma is treated with eye drops to reduce fluid
production or improve the flow of fluid from the eye, or by laser treatment or
an operation to open the drainage channels.
