Defects of Vision & Dispersion of Light | Science Class 10 PDF Download

What is colour blindness?

Colour blindness is a vision problem where a person cannot tell certain colours apart. It happens because of a defect in the eye that makes it hard to distinguish between specific colours.

Colour blindness is usually inherited from parents to children. The human retina has three types of cone cells that are sensitive to different lights: blue, green, and red. If someone lacks a particular type of cone, they won't be able to see certain colours. For instance, if someone doesn't have cones for blue light, they won't see the colour blue.

Defects of Vision and Their Correction

Issues with normal vision are referred to as defects of vision. Common refractive defects include:

• Myopia or shortsightedness
• Hypermetropia or longsightedness
• Presbyopia

The eye's ability to focus on both near and far objects is known as accommodation. The nearest distance for clear vision without strain is called the near point of the eye, which is about 25 cm for a young adult with normal vision.

What is shortsightedness (or myopia)?

Shortsightedness (or myopia) is when the eye struggles to see distant objects clearly but can see nearby objects well. A person with this issue has a far point that is closer than infinity and can see clearly only up to a few metres. In a myopic eye, the image of a distant object is formed in front of the retina instead of on it. This can happen due to:

• Excessive curvature of the eye lens
• Elongation of the eyeball

This defect can be corrected with a concave lens of the appropriate power.

What is hypermetropia?

Hypermetropia, or farsightedness, allows a person to see distant objects clearly while nearby objects appear blurry. The image of nearby objects focuses beyond the retina. This can be corrected with a suitable convex lens.

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What is presbyopia?

Presbyopia is a common issue that occurs with age, where the eye loses its ability to focus on nearby objects.

Some people may have both myopia and hypermetropia. These individuals often need bi-focal lenses, which include both concave and convex lenses. The upper part is for distant vision, while the lower part is for near vision.

Other vision issues, like cataracts, can lead to partial or complete vision loss due to clouding of the crystalline lens. This condition is often treated with surgery.

Causes of shortsightedness (or myopia) Myopia or shortsightedness is caused by the following reasons:
(a) Decrease in focal length of the eye lens, i.e. the eye lens becomes more convergent.
(b) Elongation of the eyeball, i.e. the increased length of the eyeball.

How is shortsightedness (or myopia) corrected?

The shortsightedness (myopia) can be corrected by making the eye lens less convergent. This can be done by placing a concave lens (divergent lens) of suitable focal length before the eye lens.

The rays of light coming from a distant object after passing through the concave (diverging) lens of the spectacles diverge slightly. As a result, the rays entering the eye appear to come from the far point of the myopic eye, and therefore get focused at the retina to form a clear image.

How to calculate the focal length and power of the lens used for correcting a myopic eye?

The corrective lens (concave lens) needed to correct a myopic eye should form the image of the far-off object (e.g. at infinity) at the far point (d) of the myopic person. Thus,

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What is longsightedness (or hypermetropia or hyperopia)?

The longsightedness (or hypermetropia) is the defect due to which the eye is not able to see clearly the nearby objects though it can see the distant objects clearly.

So, a longsighted eye has its near point farther away from the normal near point (about 25 cm for an adult)

Causes longsightedness (or hypermetropia) Hypermetropia or longsightedness is caused due to the following reasons: 
(i) Increase of the focal length of the eye lens, i.e. the eye lens becoming less convergent. 
(ii) Shortening of the eye ball, i.e. the length of the eye ball has decreased.

How is longsightedness (or hypermetropia) corrected?

• Hypermetropia, often called far-sightedness, is a condition that can be corrected with a convex lens of the appropriate power.
• A person with hypermetropia can see distant objects clearly but struggles to see nearby objects well.
• The near point for someone with this condition is further away than the typical 25 cm.
• Individuals often need to hold reading materials much further than 25 cm from their eyes for comfortable reading.
• This issue occurs because light rays from close objects focus at a point behind the retina.

This issue can happen for two reasons:

• The focal length of the eye lens is too long.
• The eyeball is too small.

Eyeglasses with converging lenses give the extra focusing power needed to create a clear image on the retina.

What is astigmatism?

Astigmatism occurs when light coming into the eye does not focus at the same point in both the horizontal and vertical planes. This condition means that a person with astigmatism cannot see clearly in all directions.

Causes of Astigmatism

Astigmatism happens when the cornea or the eye lens, or both, are not evenly curved. This uneven shape means that they are more curved in one direction than in the other, which stops light from focusing correctly on the retina, causing blurred vision.

Common Refractive Defects of Vision

• Myopia (near-sightedness)
• Hypermetropia (far-sightedness)
• Presbyopia

Knowing about these conditions is essential to understand astigmatism and how to correct it.

Myopia

In a myopic eye, the image of a faraway object is formed in front of the retina instead of directly on it. This issue can be caused by too much curvature of the eye lens or an elongated eyeball. Myopia can be fixed by wearing a concave lens with the right power.

Hypermetropia

Hypermetropia, or far-sightedness, occurs when the image of nearby objects is focused beyond the retina. For those with hypermetropia, the near point is further away than the normal near point (25 cm). This condition is correctable using a convex lens with the appropriate power.

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Presbyopia

Presbyopia is a vision condition affecting older adults, where the eye has difficulty focusing on close objects due to a reduced ability to adjust. As people age, the eye's accommodation ability declines, leading to presbyopia.

Correcting Astigmatism

Astigmatism can be treated with cylindrical lenses, which have different curvatures for horizontal and vertical directions. These lenses assist in focusing light correctly on the retina, effectively addressing the refractive error.