MP Board · Class 10 · Science · The Human Eye and Colourful WorldExplain the structure and working of the human eye with the help of a neat and labeled diagram description. Also, explain the phenomena of power of accommodation.

Structure 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. Light enters the eye through a thin membrane called the cornea.

• Cornea: It forms the transparent bulge on the front surface of the eyeball. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea.
• Iris: It is a dark muscular diaphragm that controls the size of the pupil. The iris gives the eye its distinctive colour.
• 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.
• Eye Lens: The crystalline lens simply provides the finer adjustment of focal length required to focus objects at different distances on the retina.
• Retina: It is a delicate membrane having enormous number of light-sensitive cells. The light-sensitive cells get activated upon illumination and generate electrical signals.
• Optic Nerve: These signals are sent to the brain via the optic nerve. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.

Working of the Human Eye\nLight rays from an object pass through the cornea, pupil, and eye lens. The eye lens converges these light rays to form a real, inverted, and diminished image on the retina. The light-sensitive cells of the retina get activated and generate electrical impulses. These electrical impulses are transmitted to the brain through the optic nerve. The brain interprets the messages and translates them into an upright image, allowing us to see the object clearly.

Power of Accommodation\nThe ability of the eye lens to adjust its focal length is called accommodation. The ciliary muscles play a crucial role here:

• When we look at distant objects, the ciliary muscles relax, making the eye lens thin, which increases its focal length, enabling us to see clearly.
• When we look at nearby objects, the ciliary muscles contract, making the eye lens thicker, which decreases its focal length, enabling us to see nearby objects clearly.\nThe maximum variation of the focal length of the eye lens can enable the eye to focus objects at varying distances ranging from infinity to about 25 cm (near point).