The Human Eye and the Colourful World | Class 10 Science Notes

DEFECTS OF VISION AND THEIR CORRECTION

The human eye can suffer from refractive defects when it loses its ability to focus images properly on the retina, causing blurred vision. The three main defects are myopia (near-sightedness), hypermetropia (far-sightedness), and presbyopia. Myopia occurs when distant objects appear blurred because their images form in front of the retina. This happens due to excessive curvature of the eye lens or elongation of the eyeball. The far point of a myopic eye is closer than infinity, sometimes just a few meters. It is corrected by using concave lenses, which diverge light rays so that the image forms on the retina. Hypermetropia is the inability to see nearby objects clearly because their images form behind the retina, caused by a too flat lens or a shorter eyeball. The near point is farther than normal (25 cm). Convex lenses are used to correct hypermetropia by converging light rays to focus on the retina. Presbyopia is an age-related defect where the eye loses accommodation power due to weakening ciliary muscles and less flexible lenses, making it difficult to see nearby objects. It can be corrected by bifocal lenses combining concave and convex lenses or by contact lenses and surgery. These corrections restore clear vision by adjusting the focal point onto the retina.

📊 Diagram: Figure 10.2 (a) Far point of a myopic eye; Figure 10.2 (c) Correction for myopia; Figure 10.3 (a) Near point of a Hypermetropic eye; Figure 10.3 (b) Hypermetropic eye; Figure 10.3 (c) Correction for Hypermetropic eye

🔗 Connection: Having understood vision defects and their corrections, the chapter now proceeds to study the refraction of light through a prism, which is fundamental to understanding natural optical phenomena like rainbows.