THE EYE - structure, function and defects -
and correction of vision defects - long sight and short-sight,
also cataracts - surgery and the cause of colour blindness
Doc Brown's biology exam revision study notes:
This page gives you information on the
structure and function of the eye. The function of the iris reflex,
cornea, pupil, lens focussing, action of the ciliary muscles, optic nerve,
retina sclera and suspensory ligaments are described and explained. Diagrams
explaining how vision defects
of short-sight and long-sight are corrected.
Sub-index for this page on the eye
1.
The structure
of the eye - function of structural features explained
2.
The pupil-iris reflex
action response to bright light
3.
How the eye
focuses on objects and aging effects
4.
Vision
defects and their correction using lenses (short sight and long sight)
The causes and correction of
short-sightedness (myopia)
The causes
and correction of longsightedness (hyperopia)
5.
Other treatments
for eye defects: contact lenses, laser surgery
6.
Other
eye defects - cataracts and colour blindness
See also in physics notes:
Optics - types of lenses (convex, concave, uses),
experiments and ray
diagrams, correction of eye defects
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1. The
structure of the eye
The tissues of the eye are all adapted to
fulfil their specific function - see diagram on the right.
Cornea
The cornea has a fixed shape and the transparent tough outer layer of the sclera at the front
part of the eye. It collects and refracts (bends slightly) the light into the
interior of the eye.
Iris
The iris acts as the aperture (opening like
in a camera) and is the coloured part of the eye. The iris has radial and circular muscles that can control the diameter of the
aperture and so controls how much light enters the eye (see iris reflex
in next section).
Lens
The lens is an adjustable biconvex lens,
it can change shape - it also further
refracts light and focuses
the light rays onto the retina - the layer of receptor cells called rods
and cones (see below) create the information for the brain to produce an
image in your mind.
Retina
The retina is the light sensitive layer of photo-receptor cells
called rods and cones, that react to the stimulus of visible light photons -
they respond to both colour (wavelength) and intensity (brightness) of the light
rays by creating nerve impulses sent to the brain from the image brought
into focus on the retina.
The rods are more sensitive in dim
light, but cannot distinguish colours.
The cones are sensitive to different
colours but are not as effective in dim light.
Cone cells connect to
individual nerve cells (neurones) which enables us to view objects
in acutely fine detail.
As you move away from the eye's central
vision, there are fewer cones and more rods, so our peripheral
vision is less acute and in black and white (see note on fovea
below).
There are three types of cone cells and
each type detects and absorb different colour ranges e.g. mainly red, green or blue photons of
visible light for colour vision.
Combinations of the cell responses create
signals to the brain which it interprets into all the colours of the visible we see
- violet, indigo, blue, green, yellow, orange and lots of shades
in between!
and, things are even more
complicated than that.
e.g. yellow light is
detected by both red and green cones, so you don't really
see a pure yellow colour.
For more on the
physics of colour
(see GCSE school physics notes)
The
fovea is central area on the retina responsible for
high resolution vision it is much more densely
populated with cone photoreceptors than the more peripheral
regions of the retina.
Optic nerve
The optic nerve carries the nerve impulses
produced by the retinal cells to the brain which creates the image you
perceive. So, the information (light from the image) is converted into
electrical impulses by the receptor cells and transmitted to the brain
along the optic nerve and the brain processes the nerve impulses into
the image you perceive.
Pupil
The pupil is the hole in the centre of the iris
that lets light through,
whose size is controlled by the relaxing and contracting of the iris.
Sclera
The sclera is the tough supporting wall of the
eye.
Vitreous humour
The vitreous humour the liquid filling most
of the volume of the eye ball.
Conjunctiva
The conjunctiva is a thin mucous membrane that lines the inside
of the eyelids and covers the sclera (the white of the eye) and
helps prevent microorganisms from entering the eye.
Suspensory ligaments and ciliary
muscle
The ciliary muscles and suspensory ligaments
work together to control the shape of the lens in order to focus an image on the
retina - see later section on how the eye focuses on near and distant
objects.
These two systems must work together to alter
the shape of the lens and focus an image on the retina.
The ciliary muscles form a circular ring
around the lens and can contract and relax like any other muscles
Attached to the ciliary muscle are the suspensory
ligaments that hold the lens in place, and they can be stretched or
loosened - this is important for the eye to be able to focus on near
or far objects.
When the ciliary muscles contract the sensory ligaments
relax to make the lens thicker to bring close objects into focus.
When the ciliary muscles relax the sensory ligaments
contract to make the lens thinner to bring distant objects into
focus.
Obviously, this gives a wide range of focussed distances from
very close objects to very distant objects, but this focussing range
tends to narrow as you get older.
Blind spot
There is small area on the retina where the optic
nerve connects.
In this area there are no light-sensitive cells
(rods or cones) so on this part of your retina you can't see
anything.
This area is called the blind spot.
Extra chemical protection of
the eye.
Our eyes produce a
chemical called lysozyme in tears, that kills bacterial microorganisms on the surface
of the eye. Lysozymes are enzymes that
break down the cell walls of bacteria, so destroying the
bacteria on the surface of the eye. Lysozymes are found in
several secretions produced by the body.
See
more on our body defences
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sub-index
2. The
pupil-iris
reflex action
Very bright light can damage the retina of your eye.
Fortunately, you have a automatic (involuntary) reflex action for protection against
intense light into your eye - the pupil-iris reflex.
The
diagram shows what happens when your eye experiences bright light (pupil
gets smaller) and in dim
light (pupil gets larger).
Exposed to an average light intensity, the pupil would
be a size intermediate to that shown in the right diagrams.
When your visible light receptors on your retina detect very bright
light a reflex arc is triggered that contracts the pupil, reducing
its diameter to let less light in.
(a) Reflex arc: When light receptor cells in the eye detect very
bright light (high intensity), a message is sent along a sensory neurone to
the brain (CNS).
(b) The message then travels along a relay
neurone to a motor neurone, which informs the circular muscles of the iris
to contract to make the pupil smaller (see diagram on right, left figure).
(c) The radial muscles then relax and the smaller pupil lets less
light through to the light sensitive retina at the back of the eye.
Note the antagonistic action of circular and radial muscles in
the iris
(d) When your visible light receptors on your retina
detect dim light a reflex is triggered that expands the pupil,
increasing its diameter to allow more light in.
To make the pupil
larger the circular muscles of the iris relax and the radial muscles
contract - tighten.
In terms of the reflex arc: When light receptor
cells in the eye detect the light is dim (low intensity), a message is
sent along a sensory neurone to the brain (CNS). The message then
travels along a relay neurone to a motor neurone, which informs the
circular muscles of the iris to relax to make the pupil larger to let
more light into the eye (see diagram above on right).
(e) Because of the iris reflex action, your
eye can automatically adjust to any light conditions.
(i) Its useful for your eye's protection against
potentially harmful bright light.
Note that your first 'instinct' is to close
your eyes, but except for exceptionally bright light, your eyes will
adjust from dimmer to brighter conditions - you don't normally
notice the time lapse.
(ii) It is also equally useful in dim light when
you still need to be able to see without knocking into things!
Note there is a short time lapse as your eyes
adjust to the dimmer light conditions, and here, you do actually
notice your eye's adjustments as things gradually become clearer.
For more on reflex arc see
An introduction
to the nervous system including the reflex arc
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3. How the eye
focuses on objects and ageing effects
To be as effective as it can be, the must be able
to focus the light onto the retina from both near and distant objects.
The eye does this by changing the shape of the
convex
lens to increase or decrease its refracting power this is achieved by
the ciliary muscles and suspensory ligaments - which together to alter
the shape of the lens and focus the image on the retina.
This ability of the lens to change shape and focus
on near or far objects is called
accommodation.
You should have dealt with the
focussing power of convex lenses
in the physics lessons.
(i)
Focussing on near objects
Rays from a near object are diverging and the
eye needs to make an appreciable adjustment to focus the image.
When the object is near to the eye the ciliary
muscles contract and this relaxes the suspensory ligaments - which
can slacken and allow the lens to become more curved - more
'rounder', 'fatter' or 'thicker'.
This increases the refracting power of the eye lens and the
light rays are refracted more to bring them into focus onto the
retina at the back of the eye.
(ii)
Focussing on distant objects
Rays from a distant object are almost parallel and
the eye only needs minimum adjustment to focus the image.
When the object is much longer distance away,
the ciliary muscles relax, allowing the suspensory ligaments to
tighten (contract) - this makes the lens become less curved or more 'thinner'.
This decreases the power of the lens and the
light rays are refracted less to bring them into focus onto the
retina at the back of the eye.
(iii)
What happens as you get older!
Having read (a) and (b) you should realise
that it is the great flexibility of the eye ciliary muscles
and suspensory ligaments that allow the
eye to change the shape of its lens.
Unfortunately, as you get older, these muscles
and ligaments become less flexible and lens cannot shape as easily i.e.
interchanging between focussing on near and distant objects.
This leads to lack of focus and explains why
many older people need to use reading glass, usually of increased
power - and this includes me, so must read on in the next section!
Also, as we age, the eye changes from white to
yellow.
Also, as we get older, bright light and uv can
damage the lens.
You should wear sunglasses to protect your
eyes in bright light and uv light in particular can hasten the onset
of cataracts.
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sub-index
4. Vision
defects and their correction
Copied and re-edited from
Optics - lens types, ray
diagrams, correction of eye defects (gcse physics)
Using lenses to correct for eye
defects
The eye contains a convex lens that,
ideally, focuses the incoming light rays to produce a sharp image on the
retina at the back of the eyeball. The light receptors in the retina then
transmit the electrical signals to the brain giving you vision. However, the
eye does not always sharp images, but it is possible to correct these
defects using glass lenses.
Cause of vision defects
A malfunction of the eye in some way to
give blurred vision because the incoming light rays do not form a sharp
image on the retina.
1a. Causes of short-sightedness
(the medical term for short sightedness is myopia)
Short sighted people can't
focus correctly on distant objects.
There are many 'biological'
causes to make a person short-sighted, but here we are most
concerned with optics of the situation and how to use concave lenses
to correct the defect.
There are two principal
reasons why a person can suffer from short-sightedness (both
indicated on the diagrams).
The eyeball can be too long so
the image is formed in front of the retina.
Distant objects will seem
blurred, though close objects may be in focus.
The same effect is caused if the
lens is the wrong shape i.e. it is too powerful in refraction - too thick
or to rounded - with too short a principal focus,
so your vision is blurred.
This can be corrected with a
diverging concave lens (see the diagrams and explanation below).
You should have dealt with the
behaviour of concave lenses
in physics lessons - refraction diagram 3 below.
1b. Correcting for
short-sightedness (short sight diagram above)
Reminder: Short sighted people
can't focus correctly on distant objects.
The eyeball is too long or the
lens to thick (too strong) to produce a sharp image on the retina.
Prior to correction, the focussed image is
formed in front of the retina.
In order to move the image back
to give it a sharp focus on the retina you need to diverge the rays.
This is done with a diverging concave lens in front of the eye
- note the shape of the concave lens - curves inwards - thinner in the
middle.
The diverged rays are then
brought to a focus further back by the eye lens onto the retina.
The diagrams (1) show the normal
correct function of the eye, the effect of short-sightedness and the
correction produced by the concave lens.
2a. Causes of
longsightedness
(the medical term for long sightedness is hyperopia)
Long sighted people can't
focus correctly on near objects.
There are many 'biological'
causes to make a person long-sighted, but here we are most concerned
with optics of the situation and how to use convex lenses to correct
the defect.
There are three principal
reasons why a person can suffer from short-sightedness
(both indicated on the diagrams).
The eyeball may be too short so
the image is formed behind the retina.
Distant objects might be in focus
but any relatively close objects will appear blurred.
The same effect is caused if the
lens is the wrong shape - too weak - too thin - with too long a principal focus, so
your vision is blurred.
Long-sightedness can also be
caused if the cornea is not curved enough (not indicated on the
diagram).
This can be corrected with a
converging convex lens (see the diagrams and explanation below).
You should have dealt with
focussing power of convex
lenses in physics lessons - refraction diagram 5 below.
2b. Correcting for longsightedness
(long sight diagram above)
Reminder: Long sighted people
can't focus correctly on near objects.
The eyeball is too short or the
lens to thin (too weak in refraction) to produce a sharp image on the retina.
Prior to correction, the image is
formed behind the retina.
In order to form a sharp image on
the retina, you to use a converging convex lens in front of the eye
to bring the rays to a focus further forward - note the shape of the
convex lens - curves outwards - fatter in the middle.
The combined converging power of
the glass lens plus the eye lens bring the rays to a focus on the
retina on the inner surface of the eyeball.
The diagrams (2) show the normal
correct function of the eye, the effect of long-sightedness and the
correction produced by the convex lens.
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5.
Other more
recent treatments for eye defects
Glass lenses have been used for at
least a 1000 years to correct eye vision defects.
Three examples are described, more or
less in historical order from the 20th into the 21st century.
(i) Contact lenses - can
correct long sight or short sight problems
Contact lenses are very thin
lenses made from special plastics or silicone hydrogels - which give
a 'softer' more comfortable contact. The silicone based gels allow
oxygen to diffuse through to the cornea.
The contact lens fits onto the surface of the
eye and shaped to refract light to compensate for the natural eye
lens defect in focussing.
Advantages and disadvantages
Their popularity stems from
them being lightweight and almost invisible.
Contact lenses are far more
convenient than glasses, particularly sporting activities.
There are two main types of
contact lenses - hard and soft lenses.
Soft lenses are more
comfortable but carry a higher risk of eye infections than hard
contact lenses.
Some contact lenses are
disposed of after a day, others last longer, but require the
inconvenience of cleaning and disinfection.
(ii) Laser eye surgery - can
improve long sight or short sight
Depending on the nature of the
vision defect, poor eyesight can be sometimes corrected using laser
eye surgery.
A flap is cut in the cornea
and folded back it is then repositioned after the powerful light energy beam of the laser
vaporises tissue to change the shape of the cornea.
This changes how strongly the
cornea refracts light and the surgeon can precisely control how much
tissue is burned off to correct the vision defect.
The laser surgeon can reshape the
cornea to treat either short sightedness or long sightedness.
By
making the cornea thinner, there is less refraction (less powerful
lens action) and this helps sufferers who are short sighted. You can
also reshape the cornea to make it have a more powerful refracting
effect and this helps long sighted people.
There are always risks and
complications with any surgical procedure which includes infection
or the eye reacting adversely to the surgery so it may actually
worsen your eyesight.
(iii)
Replacement lens surgery
Long sightedness can sometimes be
treated by replacing the lens of the eye rather than reshaping the
cornea by laser surgery as described above. In the surgical
procedure the natural lens of the eye is removed and an artificial
lens, made of transparent plastic, is inserted in its place.
Replacement lens surgery is a
more tricky and higher risk procedure than laser surgery, mainly
because you are actually working inside the eyeball itself, rather
than just honing the surface of the cornea into the desired shape.
There is more risk of infection,
and, even worse, the surgical procedure may damage the retina,
potentially leading to loss of sight.
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6.
Other eye defects
(i)
Cataracts
A cataract is a cloudy patch
that forms on the lens of your eye and stops the light rays from
entering the eye normally to form a correctly focussed image on the
retina. A cataract gives you blurred vision because some of the
light is scattered. Also, colours are seen less vividly and you have
difficulty seeing in bright light.
Cataracts tend to occur in
older people because eye tissues inevitably degrade over time.
A cataract can be treated by
replacing the faulty lens with an artificial one.
In cataract
surgery, the lens inside your eye that has become cloudy is removed
and replaced with an artificial transparent plastic lens to restore
clear vision. The procedure typically is performed on an outpatient
basis and does not require an overnight stay in a hospital or other
care facility.
However, like all surgical
procedures there are potential complications e.g. infection or
damage to the retina.
(ii)
Colour blindness
Humans and related primates can
see the colours red, green and blue ('RGB') and their combinations.
Most other mammals can only
distinguish combinations of two colours..
People suffering from colour
blindness cannot distinguish certain colours - one from
another.
Red-green colour blindness is
the must common type of this disorder - they can't distinguish
red and green. The photosensitive red
and green cones in the retina of the eye are faulty and don't work
properly e.g. some people can't see red objects or magenta coloured
objects look blue (red subtracted).
Unfortunately, there is no
cure for colour blindness, because you can't replace cone cells
(at least not at the moment). However, tinted lenses can help
colour blind people distinguish colours a bit more normally.
See also in physics notes
Optics - types of lenses (convex, concave, uses),
experiments and ray
diagrams, correction of eye defects
Summary of learning objectives and key words or phrases for the eye
Be able to:
Identify the structures of the eye, including cornea,
iris, pupil, lens, retina, optic nerve and blind spot.
Describe the function of each part of the eye in terms
of the cornea – refracts light, iris – controls how much light enters
pupil, lens – focuses light onto retina, retina – contains light
receptors, some sensitive to light of different colours and optic nerve
– carries impulses to the brain.
Explain the pupil reflex in terms of light intensity and
pupil diameter.
Explain the pupil reflex in terms of light intensity and
antagonistic action of circular and radial
muscles in the iris.
Explain accommodation to view near and distant objects
in terms of the contraction and relaxation
of the ciliary muscles, tension in the suspensory ligaments, shape of
the lens and refraction of
light.
State the distribution of rods and cones in the retina
of a human.
Outline the function of rods and cones, limited to
greater sensitivity of rods for night vision and
three different kinds of cones absorbing light of different colours for
colour vision.
Identify the position of the fovea.
WHERE
NEXT?
See also in physics notes:
Optics - types of lenses (convex, concave, uses),
experiments and ray
diagrams, correction of eye defects
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