Chapter 9: Light – Reflection and Refraction
Simple, exam-focused CBSE Class 10 Science notes — star topics, concepts, common mistakes, and previous year question patterns, all on one page.
Chapter 9: Light – Reflection and Refraction
⭐ Star Topics / Most Important
- Laws of reflection and refraction
- Image formation by spherical mirrors (ray diagrams)
- Mirror formula and magnification
- Refractive index and lens formula
Detailed Chapter Notes
Reflection laws: angle of incidence = angle of reflection; incident ray, reflected ray, and normal lie in the same plane.
Concave mirror (converges light — torches, headlights, shaving mirrors) vs convex mirror (diverges light, wider field of view — vehicle mirrors).
Concave mirror image rules: object beyond C → real, inverted, diminished between F and C; at C → real, inverted, same size at C; between F and C → real, inverted, magnified beyond C; at F → image at infinity; between pole and F → virtual, erect, magnified. Convex mirror: always virtual, erect, diminished.
Mirror formula: 1/v + 1/u = 1/f. Magnification: m = -v/u = height of image/height of object. Sign convention is critical here.
Refraction is bending of light between media due to speed change, following Snell's law. The refractive index is the constant ratio of sine of incidence to sine of refraction for a given pair of media.
Lenses: convex (converging) and concave (diverging). Lens formula: 1/v - 1/u = 1/f. Power P = 1/f (metres), in dioptres — convex is positive, concave is negative.
The most common numerical mistake is ignoring sign convention entirely. Also, ray diagrams need at least two standard rays (parallel to axis, through focus, through centre of curvature/optical centre) — using just one loses marks even with a correct final image position.
Search on YouTube: Ray diagrams concave and convex mirror Class 10 tricks for a visual walkthrough of this chapter.
- An object is placed at 25 cm from a concave mirror of focal length 15 cm. Find the position and nature of the image (numerical, 3 marks)
- Why does a convex mirror always produce a virtual, erect, diminished image? (2 marks)
- Draw a ray diagram to show refraction of light through a rectangular glass slab (2 marks)
- Define refractive index. What does a refractive index of 1.5 for glass mean? (2 marks)
- The power of a lens is -2.5 D. Find its focal length and nature (numerical, 2 marks)