Do you wear glasses or contacts to see better? Many people have vision problems that require correction. If you're short-sighted, you need a diverging lens. For long-sightedness, you need a converging lens. There are other defects of vision that need different methods of correction. Let's take a closer look at the eye and the common defects of vision.

Our eyes work by light reflecting off objects and entering the eye. The different parts of the eye refract (bend) the light, so it focuses on the retina. Check out the diagram below to see how it works.

[Insert image of diagram showing how an image is formed on the retina]

In your exams, you might see the lens shown as a straight line. For more information on how we see, read our explanation on Physics of Vision.

The eye lens is made of jelly-like material that can change shape. The ciliary muscles control the shape of the lens. When the muscles relax, the lens becomes thinner, allowing you to see distant objects. When you look at something up close, the ciliary muscles contract, making the lens thicker and allowing you to see nearby objects clearly. This process is called accommodation.

Remember, defects of vision can be corrected with different methods. If you're having trouble seeing, visit an eye doctor to find the right solution for you.

Refractive defects of vision and their causes

There are three common refractive defects of vision: myopia (short-sightedness), hypermetropia (long-sightedness), and astigmatism.

Myopia, also known as short-sightedness or near-sightedness, affects a person's ability to see distant objects clearly. They can see nearby objects clearly, but anything further away appears blurry.

The far point is the point farthest from the eye where an object can be accurately focused on the retina. This means that for someone with myopia, their far point is closer to their eyes than someone with normal vision.

If you're experiencing any vision problems, it's important to see an eye doctor. They can diagnose the issue and recommend the best course of action to correct your vision.

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When someone has myopia, the image of a distant object is formed in front of the retina. This happens because the focusing system is too powerful, causing the image to come into focus before it reaches the retina. This can be due to the eye lens being too curved or the eyeball being elongated.

To correct myopia, a lens with negative power is needed to reduce the overall power of the system. This can be done using a diverging lens, which moves the focus of the image further back so that it falls on the retina. The distance between the eye lens and the corrective lens affects the power of the system. The power of a lens is measured in dioptres (D) and can be calculated using the lens equation:

Power (in D) = 1 / focal length (in meters)

Hypermetropia, also known as long-sightedness or far-sightedness, is the opposite of myopia. A person with hypermetropia can see distant objects clearly, but nearby objects appear blurry. The near point for someone with hypermetropia is further away than for someone with normal vision, which is typically 25cm.

The near point is the closest point to the eye where an object can be accurately focused on the retina. If you're experiencing any vision problems, it's important to visit an eye doctor to determine the issue and the best solution for you.

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Myopia, or nearsightedness, is a vision defect caused by the focal length of the lens being too long or the eyeball being too small. This causes rays from nearby objects to focus on points behind the retina. Myopia can be corrected using a converging lens with the appropriate power. The power of the lens needed to correct the vision of a long-sighted person with a near point of 2.5 metres can be calculated using the lens equation. The value of u is 0.25m, and the value of v is -2.5m, as this is where the image is formed. Plugging the values into the equation gives 1/f = (1/0.25) - (1/-2.5) = 10/2.5 = 4/1. Now we can use the equation for the power of a lens to calculate the power: P = 1/f = 1/4 = 0.25 dioptres. A lens correcting hypermetropia will always have positive power.

Astigmatism is when the cornea has different amounts of curvature in different planes. It can be corrected with a cylindrical lens. The prescription for a cylindrical lens consists of three pieces of information: the power needed, in dioptres, to correct someone’s short- or long-sightedness (known as the sphere or SPH); the additional power needed to correct astigmatism (known as the cylinder or CYL); and the angle to the horizontal of the plane perpendicular to plane needs correcting due to astigmatism (known as the axis or AXI).

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Defects of Vision

What are the three common defects of vision?

The three common defects of vision are myopia, hypermetropia, and astigmatism.

What are the causes of eye defects?

Eye defects are caused by the lens being too curved or not curved enough. They are also caused by the eyeball being too long or too short.

What are defects of vision?

Defects of vision happen when the lens of the eye is too curved or not curved enough. Defects of vision are also caused by the eyeball being too long or too short. Examples of defects of vision are myopia, hypermetropia, and astigmatism.

How can you correct defects of vision?

You can correct defects of vision by the use of lenses in front of the eye. The lens will be converging, diverging, or cylindrical depending on the vision defect.

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