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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.

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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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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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.

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