Chapter 9 — Light — Reflection and Refraction

NCERT Class 10 Science Chapter 9 covers Light — Reflection and Refraction, explaining the laws of reflection, image formation by spherical mirrors and lenses, refraction, refractive index, and the power of lenses.

Chapter 9 of NCERT Class 10 Science explains how light travels in straight lines and behaves when it strikes mirrors or passes between media. It covers the two laws of reflection, image formation by concave and convex mirrors using the mirror formula (1/v + 1/u = 1/f) and magnification (m = −v/u), the New Cartesian Sign Convention, and refraction governed by Snell's law (sin i / sin r = constant). The chapter also details convex and concave lenses, the lens formula (1/v − 1/u = 1/f), and the power of a lens P = 1/f measured in dioptres.

1. 01The angle of incidence equals the angle of reflection; the incident ray, normal, and reflected ray all lie in the same plane.
2. 02For spherical mirrors, R = 2f — the radius of curvature is twice the focal length; the mirror formula is 1/v + 1/u = 1/f.
3. 03A concave mirror forms real and inverted images for objects beyond F, but a virtual, erect, and enlarged image when the object is between the pole and focus; convex mirrors always form virtual, erect, and diminished images.
4. 04Refraction is the change in direction of light when it travels obliquely from one transparent medium to another; Snell's law states sin i / sin r = constant (refractive index).
5. 05The refractive index of a medium equals the ratio of the speed of light in vacuum (3×10⁸ m/s) to the speed of light in that medium; diamond has the highest refractive index (2.42) among common materials.
6. 06A concave lens always produces a virtual, erect, and diminished image regardless of object position; the power of a lens P = 1/f is measured in dioptres (D).