CBSE · Class 10 · Science · The Human Eye and Colourful WorldExplain the structure and working of the human eye with a well-labeled description. Also, explain the terms accommodation and persistence of vision.

Structure and Working of the Human Eye

\nThe human eye is one of the most valuable and sensitive sense organs. It enables us to see the wonderful world and the colours around us. The eye acts like a camera. Its lens system forms an image on a light-sensitive screen called the retina.

Key Parts of the Human Eye:

1. Cornea: It is a thin membrane that covers the front part of the eye. Light enters the eye through the cornea. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea.
2. Iris: It is a dark muscular diaphragm that controls the size of the pupil. It gives the eye its distinctive colour.
3. Pupil: The pupil regulates and controls the amount of light entering the eye. If the light is bright, the iris contracts the pupil; in dim light, it expands the pupil.
4. Crystalline Lens: It is composed of a fibrous, jelly-like material. It provides the focused real and inverted image of the object on the retina by fine-adjusting its focal length.
5. Retina: It is a delicate membrane having an enormous number of light-sensitive cells called rods and cones. Rods respond to the intensity of light, while cones respond to colours.
6. Optic Nerve: It transmits electrical signals generated by the photoreceptor cells on the retina to the brain for processing.

Working of the Eye:\nLight rays from an object pass through the cornea, pupil, and crystalline lens. The lens focuses these rays to form a real, inverted, and diminished image on the retina. The light-sensitive cells of the retina get activated and generate electrical signals. These signals are sent to the brain via the optic nerve, and the brain interprets them, allowing us to see the object upright.

Accommodation:\nThe ability of the eye lens to adjust its focal length so as to clearly focus objects at varying distances (from infinity to the least distance of distinct vision, which is 25 cm) on the retina is called accommodation. This is achieved with the help of ciliary muscles that alter the curvature and thickness of the flexible crystalline lens.

Persistence of Vision:\nThe persistence of vision is the phenomenon by which an afterimage is thought to persist on the retina for a fraction of a second (about 1/16th of a second). If stationary images of a moving object are flashed on the eye at a rate faster than 16 per second, the eye perceives them as a continuous moving object, which is the underlying principle of motion pictures.