Explain the structure and functioning of | Class 10 Science Chapter The Human Eye and the Colourful World, The Human Eye and the Colourful World NCERT Solutions

Human eye is the organ of sight. It enables us to see the wonderful world and the colours around us. The study of structure, functions and diseases of the eye is known as ophthalmology. Eyes are situated in deep protective bony cavities known as orbits or eye sockets of the skull. It is a hollow, spherical organ and is about 2.5 cm in diameter.

Structure of the eye: It consists of two parts- wall and contents. Wall consists of a fibrous coat, vascular coat and retina. And, contents consist of two chambers- aqueous chamber and vitreous chamber.

• Fibrous coat: It is a thick and tough coat which protects the eyeball, maintains its form and provides a firm surface for the insertion of eye muscles and contains two unequal regions- sclera and cornea. Sclera is an opaque, bluish white sheet of fibrous elastic tissue and contains collagen fibres. A small part is visible from the front, called white part of the eye. Cornea is thin, transparent, circular, fully visible from the front and composed of connective tissue which is covered from the outer side by stratified squamous epithelium; also lacks blood vessels. It contains a thin membrane from where light enters. Cornea contains curved surfaces which refracts (bends) the light rays towards the retina which leads to the image formation.
• Vascular coat: It is composed of Choroid, iris and ciliary body. Choroid lies in contact with sclera, dark brown in colour and darkens the cavity of the eyeball to prevent internal reflection of light that blurs the image. It also nourishes the retina. At the junction of sclera and cornea, the vascular coat sharply bends into the cavity of the eyeball to form a thin coloured partition called iris and it is perforated at the middle by an aperture called pupil. Behind the iris margin, the vascular coat is thickened to form a ciliary body.
• Retina: It is a very delicate coat and lines the whole vascular coat; contains three parts: optic, ciliary or iridial. The human eye forms the image of an object at the retina and also contains two layers of nervous and pigmented layers.
• Aqueous humour occurs in aqueous chamber and is watery fluid, secreted by ciliary processes; contains diffusible substances of the plasma. It is continuously absorbed into the blood and replaced.
• Vitreous humour occurs in vitreous chambers and is jelly- like substance, secreted by the retina during the development of eyes. It is not absorbed and replaced and consists of water, protein, vitamin and collagen fibres.

Working of eye: Eye resembles a photographic camera in structure as well as in working.

Accommodation is a reflex mechanism in which eyes change the focal power to make the farthest and the nearest object clearly visible on the retina. Human beings have the good power of accommodation. A normal eye can see light from objects from 25 cm to infinity. Accommodation requires the refraction of light rays when they pass from one medium to another for getting focus on the retina. Refraction occurs at the lens or the air- corneal surface and it depends on the angle of the light or distance of the object from cornea. Thus, the degree of refraction is changed by changing the convexity of the lens. And, all of this mechanism is done with the help of ciliary muscles or suspensory ligament also known as accommodation apparatus.

Near objects: When the light rays strike the eyes from the near object rays get diverged. Then greater refraction of light is required to see the near objects. To increase the refraction index, convexity of lens is increased by decreasing the tension in suspensory ligament. Loosening of suspensory ligament allows the lens to shorten by its own elasticity and then the lens becomes thicker and more convex. Thus increase in the thickness of the lens, shorten its focal length which adjusts it to focus on the near objects.

Distant objects: Reverse process occurs to see the distant objects. When we focus on the distant objects, then the eyes are said to be at rest. When the light rays strike on the eyes from distant objects, rays are parallel. At this, the ciliary muscles are fully relaxed, the suspensory ligament is under maximum tension and the lens becomes flattened.

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