Class 10 Science Chapter 11-The Human Eye and the Colorful World
Class 10 Science : Takshila Learning always designs subject matter in a way that it is easy to the Class 10 students to understand the concepts and learn how to attempt questions in CBSE Class 10 Science exams. In the previous article of CBSE Class 10th Science, we have discussed ‘the construction and working of the eye’. Now, we will learn ‘the defects of vision and their correction’.
Defects of vision and their correction
The ability to see is called vision.It is also called an eyesight.Vision is known as ‘Drishti’ in Hindi. There are three common defects of vision or defects of the eyes:
Myopia (Short-sightedness or nearsightedness)
Myopia is the defect of vision due to which a person cannot see the distant objects clearly, though he/she can see the nearby objects clearly. The far point of an eye suffering from myopia is less than infinity.
The defect of the eye called myopia is caused:-
- Due to thehigh converging power of eye lens because of its short focal length.
- Due to the eyeball being too long.
In some cases, in an eye suffering from myopia, the ciliary muscle attached to the eye lens does not relax sufficiently to make the eye-lens thinner to reduce its converging power. So due to the greater converging power of the eye-lens in amyopic eye, the image of a distant object is formed in front of the retina and hence the eye cannot see it clearly.
Myopia is corrected using spectacles containing concave lenses. The concave lens used for correcting myopia should be of such a focal length that it produces a virtual image of the distant object at the far point of the myopic eye. The whole purpose of using a concave lens here is to reduce the converging power of the eye-lens.
Calculation of Power of concave lens to correct myopia
The focal length of concave lens needed to correct myopia in a person is calculated by using the lens formula:-
Hypermetropia is the defect of vision due to which a person cannot see the nearby objects clearly,buthe can see the distant objects clearly. The near point of a Hypermetropia eye is more than 25centimeter away. Hypermetropia is just the opposite of myopia. The defects of the eye are caused:-
- Due to low converging power of eye-lens because of its large focal length
- Due to the eyeball being too short.
In some cases, the ciliary muscle attached to the eye lens becomes weak and cannot make the eye-lens thicker to increase the converging power so due to thelow converging power ofthe eye-lens in an eye suffering from hypermetropia the image of nearby objects is formed behind the retina and hence the eye cannot see it clearly.
Hypermetropia is corrected by using spectacles containing convex lenses. The convex lens used for correcting Hypermetropia should be of such a focal length that it forms a virtual image of the object at the near point N’ of the Hypermetropia eye.
Calculation of Power of concave lens to correct Hypermetropia
The focal length of convex lens needed to correct Hypermetropiaa in a person is calculated by using the lens formula
Presbyopia is the defect of vision due to which an older person cannot see the nearby objects clearly due to loss of power of accommodation of the eye. The nearest point of the old person having presbyopia gradually recedes and becomes much more than 25 cms away.
It is corrected in the same way as Hypermetropia by using spectacles having convex lenses. A person suffering from myopia as well as hypermetropia uses spectacles having bifocal lenses in which upper part consists of a concave lens used for distant vision and the lower part consists of a convex lens used for reading purposes.
The medical condition in which the lens of the eye of a person becomes progressively cloudy resulting in blurred vision is called cataract. Cataract develops when the eye-lens of a person becomes cloudy due to the formation of a membrane over it.