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

What is colour blindness?
Colour blindness is a defect of vision or defect of eye. When a person is unable to distinguish between certain colours, he/she is said to be colour blind. Thus, colour blindness is that defect of the eye due to which a person is not able to distinguish between certain colours.

Colour blindness is a genetic disorder, which occurs due to inheritance (from parents to their children).

Cause of colour blindness:
The retina of human eye consists of three different types of cone-shaped cells. Some are sensitive to blue light, some to green light, and some others to red light. Thus, each colour of light is detected only by the cones which are sensitive to that light. Sometimes, a person may not have a particular kind of cones on its retina. Then, such a person will not be able to distinguish between certain colours. For example, a person not having cones sensitive to the blue light in his/her eye will not be sensitive to blue colour.

Defects Of Vision And Their Correction:
Abnormalities in the normal vision of the eye are called defects of vision or defects of eyes.

The most commonly observed defects of vision (or defects of eyes) are:

(i) Myopia or shortsightedness or nearsightedness

(ii) Hypermetropia or longsightedness or hyperopia or farsightedness

(iii) Astigmatism

What is shortsightedness (or myopia)? 

Shortsightedness (or myopia) is the defect due to which the eye is not able to see the distant objects clearly though it can see the nearby objects clearly. So, a shortsighted or myopic eye has its far point nearer than infinity.

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,

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?
Longsightedness (hypermetropia) can be corrected by making the eye lens more convergent. This is generally done by placing a convex lens (converging lens) of suitable focal length before the eye lens. This is shown in Figure.

The rays from a nearby object (about 25 cm) after passing through the convex lens of the spectacles converge slightly. As a result, the rays entering the eye appear to come from the near point of the longsighted eye, and therefore get focussed on the retina to form a clear image.

How to calculate the focal length and power of the lens used for correcting a hypermetropic eye?
The corrective lens (a convex lens) needed to correct a hypermetropic (or longsighted) eye should form the image of the object placed at the normal near point (the least distance of distinct vision is 25 cm) at the near point of the hypermetropic person. Thus, 

v = Near point distance of the hypermetropic eye = - d
u = Near point distance for the normal eye = - D = - 25 cm. Using the lens formula,

What is astigmatism? 

A normal eye can focus all the light from any object at the same point. Sometimes, the light coming in the horizontal and vertical planes do not come to focus at the same point. As a result, the horizontal and vertical views of an object are not seen with the same clarity. Such a defect of eye is called astigmatism. A person suffering from astigmatism is not able to see in all the directions equally well .

Cause of astigmatism. Astigmatism occurs when the cornea or the eye lens or both are not perfectly spherical, i.e. the cornea or the eye lens or both are more curved in one plane than in the other.

Correcting astigmatism. Astigmatism can be corrected by using cylindrical lenses. Cylindrical lenses have different curvature in the horizontal and vertical directions.