Short-sightedness

1. Someone who is short-sighted sees near objects clearly but finds distant objects blurred.
2. The eye refracts the light too strongly for a distant object, so the rays cross and come to a focus in front of the retina.
3. By the time the light reaches the retina the rays have spread apart again, so each point on the object lands as a small blurred disc rather than a point.
4. One cause is an eyeball that is too long from front to back, so the retina sits behind the point where the light meets.
5. The other cause is a lens that is too curved, or ciliary muscles that cannot relax enough to flatten it, so the lens refracts light more than it should.
6. Near objects are still sharp because their light arrives spreading out, and the extra refraction is exactly what that light needed.
7. The fix has to reduce the total refraction, so a concave (diverging) lens is placed in front of the eye.
8. A concave lens is thinner in the middle than at the edges, so it spreads the rays out before they reach the cornea and the focus moves back onto the retina.

Long-sightedness

1. Someone who is long-sighted sees distant objects clearly but finds near objects blurred.
2. The eye refracts the light too weakly for a near object, so the rays are still converging when they hit the retina and would have met behind it.
3. One cause is an eyeball that is too short, so the retina sits in front of the point where the light would meet.
4. The other cause is a lens that has become too flat or too stiff, so it cannot bulge into the fat shape a near object needs.
5. The lens stiffens gradually through life, which is why long-sightedness becomes more common with age.
6. The fix has to add refraction, so a convex (converging) lens is placed in front of the eye.
7. A convex lens is thicker in the middle than at the edges, so it starts bending the rays together before they reach the cornea and the focus moves forward onto the retina.

Correcting focus with lenses

1. Spectacles hold the corrective lens a short distance in front of the eye, and they are cheap, easy to change and carry no risk.
2. They can be inconvenient for sport, they steam up, and they change how a person looks, which some people dislike.
3. Contact lenses sit directly on the surface of the cornea, so they move with the eye and give a wider field of clear vision.
4. They need careful cleaning and handling, because dirt trapped underneath can scratch the cornea or cause an infection.
5. The strength of a lens is written as a power in dioptres, and it is negative for a concave lens and positive for a convex one.
6. A prescription of $-2.5D$ therefore belongs to a short-sighted eye, and $+2.5D$ to a long-sighted one.

Surgical ways of correcting focus

1. Laser eye surgery uses a laser to remove a thin layer of tissue and reshape the cornea, which changes how strongly the cornea refracts light.
2. Flattening the cornea reduces refraction and corrects short sight, while steepening it increases refraction and corrects long sight.
3. The result is usually permanent and removes the need for glasses, but the surgery carries a risk of infection, of dry eyes and of over-correction, and it cannot be undone.
4. A replacement lens can be fitted instead, with the natural lens removed and an artificial one of the correct power put in its place.
5. That operation corrects strong defects that a laser cannot, but it is more invasive and carries a greater risk of damaging the retina.
6. Deciding between the options means weighing cost, convenience and the size of the risk, and the same choice does not suit everyone.

Colour blindness

1. Colour blindness is not a focusing problem at all, because the light reaches the retina perfectly sharply.
2. It is caused by one or more types of cone cell being missing or not working, so the brain never receives the impulses from that type.
3. The commonest form is red-green colour blindness, where the red-sensitive or green-sensitive cones do not respond normally.
4. Because the brain works out colour from the mixture of impulses from the three cone types, losing one type makes certain colours look the same as each other.
5. The condition is inherited and affects far more males than females, and it is present from birth rather than developing later.
6. There is no cure, because the faulty cone cells cannot be repaired or replaced and a lens cannot create impulses that were never produced.
7. People manage it instead, using tinted filters that make confusable colours look more different, and using labels, patterns or numbers rather than colour alone.
8. It is diagnosed with a set of test plates covered in coloured dots, where a number can only be picked out if the relevant cones are working.

Comparing the three defects

1. Short sight and long sight are focusing defects, caused by the shape of the eyeball or the lens, and both can be corrected.
2. Colour blindness is a receptor defect, caused by faulty cone cells in the retina, and it can only be managed.
3. That difference explains why a lens helps with two of them and not with the third.

Living with an eye defect

1. Uncorrected short sight makes reading a board or a road sign difficult, which is why school and driving eye tests exist.
2. Uncorrected long sight makes close work tiring, because the ciliary muscles stay contracted as they try to fatten the lens.
3. Colour blindness rules out a small number of jobs, such as some roles in aviation and rail work, where colour signals must be read correctly.