Chapter 9 — Light — Reflection and Refraction

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

• Reflection and image formation by mirrors — Light reflects so that the angle of incidence equals the angle of reflection. Spherical mirrors use this to form images, described by the mirror formula and magnification, with concave and convex mirrors giving different image types.
• Refraction and bending of light — When light passes obliquely between media, it changes direction — refraction — governed by Snell's law. The refractive index compares light's speed in vacuum to its speed in a medium, explaining why some materials bend light more.
• Lenses and their power — Convex and concave lenses form images described by the lens formula. The power of a lens, the reciprocal of its focal length measured in dioptres, quantifies how strongly it converges or diverges light.

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).