2.4.1 Structure and function of the eye

The Eye Is a Sense Organ

1. The eye is the sense organ that detects light, which is the stimulus it responds to.
2. Its job is to focus light onto the retina and to convert that light into electrical impulses.
3. The impulses travel along the optic nerve to the brain, where they are interpreted as an image.
4. The sclera is the tough white outer coat that protects the eyeball and keeps its shape, and the choroid beneath it is a dark layer that absorbs stray light and supplies blood.

The Cornea

1. The cornea is the transparent layer at the front of the eye, curved outwards like a dome.
2. It refracts the light entering the eye, bending the rays inwards so that they converge.
3. It does most of the focusing, because the change in speed as light passes from air into the cornea is large.
4. Its curvature is fixed, so it cannot adjust for objects at different distances.
5. It has no blood vessels, which keeps it transparent, and it takes oxygen directly from the air instead.

The Lens and Accommodation

1. The lens sits behind the pupil, and it is transparent and flexible so it can change shape.
2. It carries out the fine focusing that the fixed cornea cannot do, and this change of shape is called accommodation.
3. The lens is held in place by suspensory ligaments, which are attached to a ring of ciliary muscles.
4. Focusing on a near object. The ciliary muscles contract, the ring gets smaller, the suspensory ligaments slacken, and the lens becomes thicker and more curved so it refracts light more strongly.
5. Focusing on a distant object. The ciliary muscles relax, the ring gets wider, the suspensory ligaments are pulled tight, and the lens is stretched thin so it refracts light less.
6. Light from a distant object arrives almost parallel and needs little extra bending, while light from a near object is spreading out and needs much more.

The Iris and the Pupil Reflex

1. The pupil is the hole in the middle of the iris that lets light into the eye, and it is not a structure in itself.
2. The iris is the coloured ring of muscle that changes the size of the pupil, controlling how much light reaches the retina.
3. It contains two sets of muscle: circular muscles that run around the pupil and radial muscles that run outwards like the spokes of a wheel.
4. In bright light the circular muscles contract and the radial muscles relax, so the pupil gets smaller and less light enters.
5. In dim light the radial muscles contract and the circular muscles relax, so the pupil gets larger and more light enters.
6. This is a reflex action coordinated by the brain, and its function is to protect the retina from being damaged by bright light.

The Retina

1. The retina is the light-sensitive layer lining the back of the eye.
2. It contains the receptor cells, which are the rods and the cones.
3. When light hits a receptor cell it is converted into an electrical impulse, and the impulses leave along the optic nerve.
4. The image formed on the retina is upside down and back to front, and the brain interprets it the right way up.

Rod Cells

1. Rod cells are sensitive to low light intensity, so they allow vision in dim conditions.
2. They cannot detect colour, which is why everything looks grey at night.
3. They are spread across most of the retina but are absent from the fovea.
4. They give less detailed vision than cones, because many rods share a single neurone to the brain.
5. This sharing is what makes them sensitive, since the small responses of many rods are added together to trigger one impulse.

Cone Cells

1. Cone cells detect colour and work best in bright light.
2. There are three types, sensitive to red, green and blue light.
3. The brain compares the responses of the three types to work out the colour being viewed.
4. Cones give highly detailed vision, because each cone tends to have its own neurone to the brain.
5. They need bright light to respond, which is why colours cannot be seen in a dark room.

The Fovea and the Blind Spot

1. The fovea is a small pit at the centre of the retina, directly opposite the pupil.
2. It is packed with cone cells and contains almost no rods, so it gives the sharpest and most detailed vision.
3. When you look straight at something you are moving your eyes so the image falls on the fovea.
4. The blind spot is the point where the optic nerve leaves the retina, and it contains no rods or cones.
5. No light is detected there, so a small part of the field of view is missing, and the brain fills the gap using information from the other eye and the surroundings.

Investigating Responses of the Eye

2.4.2 Eye defects and their correction

Why Vision Becomes Blurred

1. A sharp image is formed only when light is brought to a focus exactly on the retina.
2. If the light converges in front of the retina or is still converging behind it, the image on the retina is blurred.
3. This happens when the eyeball is the wrong length or when the cornea or lens refract by the wrong amount.
4. Most defects are corrected by placing a lens in front of the eye that changes the light before it reaches the cornea.

Short-Sightedness

1. A person with short-sightedness, also called myopia, can see near objects clearly but distant objects look blurred.
2. Light from a distant object is brought to a focus in front of the retina.
3. This happens because the eyeball is too long, so the retina sits further back than the focal point.
4. It can also happen because the cornea is too curved or the lens is too thick, so the light is refracted too much.
5. It often develops during childhood and adolescence as the eyeball grows, and it tends to run in families.

Correcting Short-Sightedness

1. Short sight is corrected with a concave lens, also called a diverging lens, which is thinner in the middle than at the edges.
2. The lens spreads the light rays out before they enter the eye.
3. The cornea and lens then bring them to a focus further back, so the focal point moves onto the retina.
4. The same correction can be worn as a contact lens, which sits directly on the cornea and gives a wider field of view.
5. Laser eye surgery corrects it permanently by removing tissue to flatten the cornea, so the cornea refracts the light less.
6. A replacement lens can be implanted inside the eye for a very severe case, though this carries a higher risk of infection and damage to the retina.

Long-Sightedness

1. A person with long-sightedness, also called hyperopia, can see distant objects clearly but near objects look blurred.
2. Light from a near object is brought to a focus behind the retina.
3. This happens because the eyeball is too short, so the retina sits in front of the focal point.
4. It can also happen because the lens is too thin or not curved enough, so the light is not refracted enough.
5. It becomes very common after about the age of 40, because the lens gradually loses its elasticity and can no longer become thick enough to focus on close work.

Correcting Long-Sightedness

1. Long sight is corrected with a convex lens, also called a converging lens, which is thicker in the middle than at the edges.
2. The lens brings the light rays together before they enter the eye.
3. The cornea and lens then bring them to a focus further forward, so the focal point moves onto the retina.
4. Contact lenses work in the same way, and reading glasses are simply convex lenses used only for close work.
5. Laser surgery can also be used, this time to make the cornea more curved so that it refracts light more strongly.

Cataracts

1. A cataract is a cloudy patch in the lens of the eye.
2. It forms as protein builds up in the lens, usually with age, and clumps together so the lens is no longer fully transparent.
3. Light is scattered rather than passing straight through, so the image on the retina is blurred and misty.
4. Colours look faded, bright lights produce a glare or a halo, and vision is worse at night.
5. The risk is increased by ageing, by long exposure to ultraviolet light, by smoking and by diabetes.
6. A cataract is corrected by surgery to remove the cloudy lens and replace it with an artificial plastic lens.

Colour Blindness

1. Colour blindness is a defect in which certain colours cannot be told apart.
2. It is caused by one or more types of cone cell in the retina not working properly or being absent.
3. Red green colour blindness is by far the most common form, and it affects the red or the green cones.
4. It is almost always inherited rather than developed, and it is much more common in males than in females.
5. There is no cure, because the faulty cone cells cannot be repaired or replaced.
6. Tinted lenses and filters can help some people tell certain colours apart, but they do not restore normal colour vision.

Comparing the Four Defects

1. Short-sightedness. Distant objects blur because light focuses in front of the retina, and a concave lens corrects it.
2. Long-sightedness. Near objects blur because light focuses behind the retina, and a convex lens corrects it.
3. Cataracts. All vision becomes cloudy because the lens scatters light, and replacing the lens corrects it.
4. Colour blindness. Colours cannot be distinguished because cone cells are faulty, and it cannot be corrected.