Question

In a healthy human eye, the focusing is done by the
A. To and fro movement of the eye-lens
B. Changing the curvature of the retina
C. Change in the convexity of the lens through ciliary tubes
D. Change in the refractive index of the eye fluid.

Answer 1

Hint – For solving this question we must know about the structure of the human eye. So, we will start by writing about the structure of the human eye and functioning of its different parts. After that we will get the required answer.

Complete step-by-step solution -
Structure and working of human eye: The main parts of the human eye are as follows:
1. Sclerotic – It has a tough and opaque white covering, called sclerotic which protects and holds the eyeball.
2. Cornea – It is a transparent membrane on the front portion of the eyeball through which light enters the eye.
3. Choroid – It is a black membrane below the sclerotic. It absorbs stray light and avoids any blurring of the image due to multiple reflections in the eyeball.
4. Iris and pupil – Iris is an opaque circular diaphragm having a small central hole called the pupil. Under the muscular action of the iris, the size of the pupil becomes smaller in bright light and larger in dim light.
5. Eye lens – It is a double convex lens situated behind the iris. It is composed of a fibrous, jelly like material. The lens is held in position by suspensory ligaments and connected to the sclerotic by the ciliary muscles.
6. Retina – It is a delicate membrane of nerve fibres on the inner side of the back wall of the eye. It contains sensitive cells called rods and cones. Rods are sensitive to intensity of light while cones are sensitive to colours.
7. Blind spot and yellow spot – In the region where the optic nerve enters the eyeball, there are no rods and cones. This region is totally insensitive to light and is called blind spot.
8. Aqueous humour and vitreous humour - Aqueous humour is a salty fluid that fills the space between the cornea and the eye lens. Vitreous humour is a jelly like fluid that fills the space between the retina and the eye lens.

Whenever tension is applied on the eye lens by the ciliary muscles, i.e., whenever it contracts or relaxes, the shape or the curvature of the eye lens changes, hence its radius also changes. As we know that radius of the lens is related to the focal length of the lens by the relation $R = 2f$, where $R$ is the radius of the lens and $f$ is the focal length of the lens. Therefore, whenever radius is changed focal length is also changed which further changes the focus. This enables the eye lens to focus the image of the objects at different distances on the retina of the eye.
These ciliary muscles form ciliary tubes, thus focusing is done by ciliary tubes.

Hence, option C is the right answer.

Note – The human eye is an organ that reacts to light and allows vision. Rod and cone cells in the retina allow conscious light perception and vision including colour differentiation and the perception of depth. The human eye can differentiate between about 10 million colours and is possibly capable of detecting a single photon.