Question

The defect of the eye develops in old age because
A. Lens becomes opaque
B. Eyeball become small
C. Eyeball become long
D. The lens loses its elasticity

Answer 1

Presbyopia is the defect of old age in which both the near and distant objects have blurred. hence the accommodation power diminishes.

Complete answer:
1. The Human eye works on the refraction of light through a natural convex lens made up of transparent living material and enables us to see things around us.
2. And the ability to see is called vision, eyesight, or Drishti. The human eye consists of the cornea, iris, pupil, ciliary muscles, eye lens, retina, and optical nerve.
3. The front part of the eye called the cornea is made of a transparent substance and its outer surface is convex in shape. It is through the cornea that the light coming from objects enters the eyes.
4. Just behind the cornea is the iris which is also called the colored diaphragm. A hole in the middle of the iris is called the pupil. Then behind it is the eye lens which is a convex lens.
5. It is due to the support of ciliary muscles that the eye lens is held in position. The eye lens is flexible and thus can change its focal length and shape with the help of ciliary muscles.
6. The light rays coming from the object enter the eyes through the pupil and fall on the eye lens. The eye lens then converges the light rays and produces an image of the object on the retina which is real and inverted.
7. Retina has a large number of light-sensitive cells that can generate electrical signals. After the image is formed on the retina it sends electrical signals to the brain and we have a sensation of image. Also, even though the image formed on the retina is inverted, our mind interprets it as erect.
8. The function of the iris is to adjust the size of the pupil. If the amount of light entering the eye is less then the pupil expands so that more light can enter the eye and in case the amount of light entering the eye is large then pupil contracts.
9. The light-sensitive cells in the retina of our eye are of two shapes; rod shaped and cone shape. The function of rod-shaped cells is to respond to the brightness of a light.
10. And the function of cone-shaped cells is to make us see colors and distinguish between them.
11. When the rays of light are coming from distant objects they are diverging at the beginning but become parallel when they reach our eye.
12. Therefore to see a distant object, we need to have a convex eye-lens of low converging power to focus them to form an image on the retina of the eye.
13. The convex eye-lens of low converging power has a large focal length and is quite thin. When the rays of light are coming from the nearby objects they diverge when they reach our eyes.
14. Therefore, to see a nearby object we need to have a convex eye-lens of high converging power so as to focus and form an image on the retina. Convex eye-lens with high converging power have short focal length and are thick.

So, the correct answer is “Option D. The lens loses its elasticity”.

Note: The medical condition in which the lens of the eye of a person becomes progressively cloudy resulting in blurred vision. It develops when the eye-lens of a person becomes cloudy due to the formation of a membrane over it. It decreases the vision of the eye gradually and can lead to a total loss of vision of the eye. It can be restored after getting surgery. The opaque lens is removed and an artificial lens is inserted in its place via operation. This defect cannot be corrected by any type of spectacle lenses.