There are many different issues that can arise in the eye, such as long-sightedness and short-sightedness. Each of these defects has slightly different causes and somewhat different fixes.
Eye Defects Definition
Eye defects are conditions that can affect the function of the eye. The purpose of the eye includes its role as a sensory organ and its function in the Immune System. If either of these functions is affected, we can say that an eye defect is present.
Common Eye Defects
Eye defects may refer to conditions like myopia or hyperopia. These are short-sightedness and long-sightedness, respectively. These eye defects impact our vision and can be corrected through corrective eye surgeries, contact lenses or replacement lenses. Eye defects can be natural and caused by ageing. On the other hand, they can be caused by infection, trauma or as a result of other conditions such as Diabetes or stroke.
The cause of the eye defect will play a huge role in determining the extent to which the eye is damaged, which treatment is best for the eye defect, and whether it can be corrected.
Myopia
One of the most common eye defects is myopia, commonly referred to as short-sightedness. It is thought that by 2030, over half the world's population will have been diagnosed with myopia.
Short-sightedness is when people can see objects close to them clearly, but struggle to focus on things further away in the distance.
There are two leading causes of short-sightedness;
The eyeball seems to have been stretched, increasing the distance between the lens and the retina.
The lens is thickened or curved, focusing the light in front of the retina rather than on the retina itself.
Look at Figure 1 to see how short-sightedness (myopia) affects the eye.
Correction of Myopia
We can place a corrective concave lens in front of the eye to correct short-sightedness, which causes the light to refract slightly before entering the eye. This refraction allows the light to be focused on the retina.
The retina is where the rod and cone Cells are found (photoreceptors). This area of the eye is where our vision comes from. The photoreceptors found here are similar to the photoreceptors found in the chlorophyll of Plants.
Take a look at our article on Eye Anatomy to get more information on the different structures of the eye! Make sure you do not get myopia confused with hyperopia in your exams!
Old Age Eye Defect
Presbyopia is used to describe the eye defect caused by old age. This is caused by the thickening of the lens, which affects the ability of the lens to refract light onto the retina. Presbyopia is a type of hyperopia or long-sightedness. Hyperopia itself is not entirely related to old age, whereas presbyopia is wholly linked to ageing. There are a few different causes of hyperopia.
Long-sightedness is when people can see distant objects clearly but struggle to focus on things closer to them.
There are two leading causes of long-sightedness;
The eyeball is too small, causing the distance between the lens and retina to be too small.
The lens cannot become thick enough to focus due to a lack of elasticity (more common in the elderly).
Can you see how the causes are different for long-sightedness and short-sightedness? Have a look at the table in the next section, which summarises them.
Correction of Hyperopia
To correct long-sightedness, we need to use a convex lens to focus light on the retina rather than behind it. This convex lens is different from the concave lens that we use to correct short-sightedness, but both act to refract light and redirect it to the lens.
If you are curious about how these lenses refract light and work slightly differently, look at our article on 'Lenses' in Physics! You must know those concave and convex lenses are used differently! The lens's shape and thickness decide how much light is refracted once it hits the lens.
Causes of Eye Defects
In more detail, we discussed the causes of myopia (short-sightedness) and hyperopia (long-sightedness) in our sections 'common eye defects' and 'old age eye defects'. Below is a brief summary of the cause and effects of long-sightedness and short-sightedness.
Affect on | Long-sightedness | Short-sightedness |
Lens | Thickened (lack of elasticity) | Thickened or curved |
Light | Can't focus | Focuses on the front of the retina |
Eyeball | Too small | Stretched |
Distance between lens and retina | Decreases | Increases |
Old age is one of the factors that can lead to eye defects, but there are some other causes too. The most common causes of eye defects are infections, trauma, and secondary causes.
Secondary causes are things that impact vision indirectly. For example, people that have Diabetes can develop diabetic retinopathy. This is a condition where the Blood vessels surrounding the retina are damaged.
The eye can also become infected, leading to conditions like conjunctivitis - an infection of the conjunctiva. These conditions can lead to temporary or permanent eye defects. A temporary eye defect can be reversed or may even heal on its own. However, a permanent eye defect may lead to permanent blindness or impaired vision that remains for the rest of your life.
Vitamin deficiencies can also lead to eye defects. Vitamin A, B12, C, D, and E are super important for our eyes. We can get these vitamins through our diet and sunlight (for vitamin D).
Diagram of Eye Defects
It's essential we can visualise how different defects affect the eye. Each fault has a different effect on vision and a different way that we then use spectacle lenses to correct the defect. Have a look at the diagram below.
This is an example of a ray diagram which shows how visible light rays hit the back of the eye at different locations.
Lenses Used for Short-Sightedness
As we have discussed, short-sightedness (myopia) is an eye defect preventing people from seeing objects far away. This is because the focus point of light is before the retina due to early refraction. By using a concave lens, we can allow light to hit the retina rather than stopping early.
Short-sighted people will wear spectacles that have concave lenses.
Lenses Used for Long-Sightedness
People who are 'long-sighted' (have hyperopia) can see distant objects clearly, but struggle to see close-up things. In this case, the eye refracts light rays and focuses them beyond the retina. To correct this, we use a convex lens allowing the light rays to land on the retina rather than beyond it after refraction.
Long-sighted people will wear spectacles that have convex lenses.
Concave refers to a shape that is hollowed out or rounded inward.
Convex refers to a curved shape or rounded outward.
Correction of Eye Defects
Contact lenses, laser eye surgery and replacement lenses are all potential corrections of eye defects. The different treatments and corrections of eye defects each have their advantages and disadvantages, as well as varying times when they can be used.
Contact Lenses
Contact lenses are clear lenses that sit on top of the eye. These can be split up into hard and soft contact lenses.
An advantage of contact lenses is that they enable people with eye defects to participate in sports.
A disadvantage of soft lenses is that they carry more risk of infection although they are more comfortable than rigid lenses.
Laser-Eye Surgery
We can use laser eye surgery to correct eye defects. This surgery aims to change the shape of the cornea. Doing so can change how light is refracted onto the retina.
Again, to understand this correction fully, you must understand the Eye Anatomy!
Laser-eye surgery is slightly different for myopia (short-sightedness) than for hyperopia (long-sightedness). In myopia, the cornea is made thinner to reduce its refractive power. In hyperopia, we increase refractive power by changing the shape of the lens.
An advantage of laser eye surgery is that it replaces the need to wear glasses and contact lenses, which is often desirable.
A disadvantage of laser eye surgery is that there is a risk of further, potentially permanent damage to the eye due to the surgery.
Replacement Lens Surgery
The final correction that we will discuss is lens replacement surgery. Lens replacement surgery completely replaces the lens of the eye with a plastic artificial lens.
An advantage of this surgery is that it often corrects eye defects quickly and painlessly and means the patient no longer has to wear glasses or contact lenses. This procedure tends to be more expensive than going for laser-eye surgery or wearing contact lenses due to the high quality of lenses used.
A disadvantage of this surgery is that it is more invasive than laser surgery (which only changes the shape of the lens). Therefore, it carries a risk of damage occurring to the retina leading to sight loss, similar to the potential damages that can come from laser eye surgery.
It is essential that you can discuss these different corrections along with the advantages and disadvantages of each!
Defects of the Eye - Key takeaways
- Myopia is short-sightedness
- Hyperopia is long-sightedness
- Myopia and hyperopia have many different causes
- Contact lenses, laser eye surgery and replacement lenses can correct eye defects. These have their advantages and disadvantages.
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