The eye contains light-sensitive receptor cells called rods and cones in the retina. Light enters through the pupil and is focused by the cornea and lens onto the retina. The iris controls the size of the pupil to regulate the amount of light. The retina converts light signals into nerve impulses sent to the brain via the optic nerve to form images. Common vision problems include nearsightedness and farsightedness caused by the eyeball or lens being an abnormal shape and astigmatism caused by irregularly curved surfaces of the cornea or lens. Aging can cause the lens to lose elasticity and clarity leading to difficulties with accommodation and conditions like cataracts and glaucoma.

1. The Eye
The receptors are the parts of the organism’s body that detects stimuli such as cells
or nerve endings. In animals, the receptors are usually part of a sense organ. For example,
your eye is a sense organ and the rod and cone cells in the retina are receptors and they are
sensitive to light.
The part of the eye that contains receptors cells is called the retina which is the part
of the eye that is sensitive to light. The rest of the eye serves to protect the retina or to
focus light on to it. Each eye is set in a bony socket in the skull called the orbit and the only
part of the eye not surrounded by bone is the very front.
The front of the eye is covered by a thin, transparent membrane known as the
conjunctiva which serves to protect the eye the parts behind it. It is kept moist due to a
fluid secreted by the tear glands that contains the enzyme lysozyme which kills bacteria. On
blinking, this fluid is washed across your eye by your eyelids. The sclera is a tough outer
coat that protects the part of the eye inside of the orbit. The retina is located at the back of
the eye and it contains two types of receptor cells known as rods and cones. Rods are
sensitive to dim light but only allow you to see in black and white whereas, cones give
colour vision but only in bright light.
When light falls on a receptor cell in the retina, the cell sends a message via the optic
nerve to the brain which sorts out all the messages from each receptor and builds up an
image. The closer the receptors are to each other, the clearer the image the brain gets. The
fovea is the part of the retina where the receptor cells are closely packed together and is
also the part where light is focused when you look directly at an object. The cones are the
only receptor cells located in the retina while the rods are scattered further out on the
retina.
No receptor cells are located where the optic nerve leaves the retina and this is
called the blind spot. When light falls on this place, there are no receptor cells present and
thus, no messages are sent to the brain. Behind the retina is a black layer known as the

2. choroid which absorbs all the light after it had been through the retina in order to prevent
it being scattered around the inside of the eye.
In front of the lens there is a circular piece of tissue called the iris which contains
pigments used in the absorption of light to prevent it from getting through to the retina. In
the middle of the iris is a gap known as the pupil which can adjust in size. In strong light,
the iris closes in to make the pupil small which prevents too much light from entering and
damaging the retina. In order for pupil size adjustment to occur, the iris contains muscles
such as circular muscles and radial muscles. Circular muscles lie in circles around the pupil
and when they contract, they cause the pupil to get smaller. Radial muscles run outwards
from the edge of the pupil and when they contract they cause the pupil to get larger.
Focusing
In order for the brain to see a clear image, there must be a clear image focused on
the retina. For this to occur, light rays must be focused exactly onto the retina and this is
carried out via the refracting of light rays. The cornea is responsible for the bending of the
light while the lens makes fine adjustments. Not all the light rays require the same amount
of bending to focus them onto the retina. Light rays that come from a nearby object are
going away from one another (diverging) and will need to bend inwards quite strongly.
Light rays coming from an object in the distance will be almost parallel to each other and
will not require as much bending.
The shape of the lens can be adjusted to bend the light rays more; for instance, the
fatter the lens, the more it bends the light rays and the thinner the lens, the less it will bend
them. Accommodation is the term used to describe this process and allows for the focus of
light coming from different distances. The lens is held in shape by a of ring suspensory
ligaments which alters the shape of the lens by means of the ciliary muscle. When it
contracts, the suspensory ligaments are loosened and when it relaxes they are pulled tight.
The images of an object sent to the retina from the two eyes are slightly different
and the brain is able to interpret these differences so that we can appreciate the size, shape

3. and distance of the object. This allows us to get a three-dimensional picture of the object
and although the fields of vision seen by the two eyes overlap, two eyes allow a wider range
of vision.
Long Sightedness
This occurs when the eye ball is shorter from the back to front than is usual or when
the lens is too flat. Light from a distant object can be focused on to the retina but from a
close object, the focus is behind the retina because the rays are not bent enough. The use of
convex or converging lenses can help these individuals to overcome this effect.
Short Sightedness
This occurs when the lens is too curved or the eyeball is too deep from cornea to
retina and so the rays from a distant object are bent more than necessary. This image is
thus formed in the jelly in front of the retina and is blurry by the time it reaches the retina.
The use of concave or diverging lenses aid the individual to see clearly objects that are far
away.
Astigmatism
This defect is caused by the surface of the lens and/or the cornea being irregularly
curved. This causes the light rays reaching the eye to focus in one plane but not the others.
This condition may be corrected by the usage of cylindrical lenses which only work on one
axis only. If the individual is also long sighted or short sighted, the glasses need to be a
combination of cylindrical and spherical surfaces,
Effects of Aging on Vision
As the individual ages, the lens start to lose some if its elasticity and the ciliary
muscles start to weaken. Thus, accommodation begins to become difficult.
In some cases, the lens becomes opaque and this condition is known as cataract and
thus, light is unable to pass through to the retina, stopping the individual from seeing. This

4. condition may be corrected by surgery where either the lens is removed and replace with
an artificial lens or a new lens is inserted inside the eye.
Glaucoma is another eye defect in which too much fluid gathers in front of the lens
and increases the internal eye pressure resulting in damage of the optic nerve. This causes
the individual’s quality of vision to decrease.