Chapter 10 — The Human Eye and the Colourful World

Chapter 10 of the Class 10 Science NCERT textbook, "The Human Eye and the Colourful World", covers the human eye's structure and defects of vision, dispersion of white light through a prism, atmospheric refraction phenomena like twinkling of stars, and why the sky appears blue due to scattering of light.

• The eye and correcting its defects — The eye works like a camera, using its lens and ciliary muscles to accommodate and focus on near or distant objects. Refractive defects — myopia, hypermetropia, and presbyopia — are corrected with concave, convex, or bi-focal lenses.
• Dispersion and the colours of light — A prism splits white light into the seven VIBGYOR colours because each colour bends by a different angle. The same dispersion by water droplets, combined with refraction and internal reflection, produces a rainbow opposite the Sun.
• Atmospheric refraction and scattering — Bending of light by the atmosphere makes stars twinkle while steadier planets do not. Scattering of shorter wavelengths by fine particles explains why the daytime sky appears blue rather than any other colour.

1. 01The human eye uses a crystalline lens and ciliary muscles to adjust focal length, enabling accommodation for objects from 25 cm (near point) to infinity (far point).
2. 02Myopia (near-sightedness) is corrected with a concave lens; hypermetropia (far-sightedness) is corrected with a convex lens; presbyopia (age-related) often requires bi-focal lenses.
3. 03A glass prism splits white light into seven colours — Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR) — because different colours bend through different angles.
4. 04A rainbow is formed when water droplets in the atmosphere refract, internally reflect, and disperse sunlight, and is always seen in the direction opposite the Sun.
5. 05Stars twinkle due to continuous atmospheric refraction of starlight, while planets do not twinkle because they appear as extended sources whose variations average out to zero.
6. 06The sky appears blue because fine atmospheric particles scatter shorter wavelengths (blue light) more strongly than longer wavelengths (red light).