MP Board · Class 10 · Science · The Human Eye and Colourful WorldExplain the structure and working of the human eye with the help of a labeled diagram. Also, explain the common defects of vision (myopia and hypermetropia) along with their corrective measures.

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 through which light enters. 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: It regulates and controls the amount of light entering the eye. If the light is bright, the pupil shrinks; if it is dim, the pupil expands.
• Eye 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 using ciliary muscles.
• Retina: It is a delicate membrane having enormous number of light-sensitive cells called rods and cones. Rods respond to the intensity of light, while cones respond to colours.
• Ciliary Muscles: These hold the eye lens in position and modify its curvature to alter the focal length for viewing objects at various distances.

Working of the Human Eye\nLight rays from an object pass through the cornea, pupil, and eye lens, undergoing refraction and falling on the retina. The light-sensitive cells get activated upon receiving light and generate electrical signals. These signals are sent to the brain via the optic nerve. The brain interprets these signals and processes the final upright image of the object.

Common Defects of Vision and Their Correction

1. Myopia (Near-sightedness):

   • Definition: A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. The image is formed in front of the retina instead of on it.
   • Causes: Excessive curvature of the eye lens or elongation of the eyeball.
   • Correction: It is corrected by using a concave lens (diverging lens) of suitable power, which diverges the incoming light rays so that they focus precisely on the retina.

2. Hypermetropia (Far-sightedness):

   • Definition: A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. The near point is farther away than the normal 25 cm.
   • Causes: Focal length of the eye lens is too long or the eyeball has become too small.
   • Correction: It is corrected by using a convex lens (converging lens) of appropriate power, which provides additional convergence to focus the image on the retina.