Light Reflection and Refraction Class 10 Notes (2026-27) — CBSE
Class 10 Science Chapter 9 notes: spherical mirrors, mirror and lens formulas, sign convention, refractive index and power of a lens, with image tables.
Light – Reflection and Refraction — Class 10 Science Notes
Chapter Snapshot
This chapter explains how light behaves when it bounces off mirrors (reflection) and bends while passing between media (refraction). You learn to trace rays and locate images for concave/convex mirrors and lenses, apply the mirror and lens formulas with a strict sign convention, and calculate magnification, refractive index, and the power of a lens.
Board relevance: almost every year this chapter gives one numerical (mirror/lens formula) and one ray-diagram or reasoning question. The New Cartesian Sign Convention decides whether your answer is right — most lost marks are sign errors.
Key Concepts & Definitions
Reflection of light — the bouncing back of light from a surface. Laws of reflection: (1) the angle of incidence equals the angle of reflection (∠i = ∠r); (2) the incident ray, reflected ray, and the normal at the point of incidence all lie in the same plane. These hold for every reflecting surface, including curved mirrors.
Spherical mirror — a mirror whose reflecting surface is part of a sphere.
- Concave mirror — reflecting surface curves inward (converging).
- Convex mirror — reflecting surface bulges outward (diverging).
Terms for spherical mirrors:
Term Meaning
Pole (P) Centre of the mirror's reflecting surface
Centre of curvature (C) Centre of the sphere the mirror is part of
Radius of curvature (R) Distance PC; radius of that sphere
Principal axis Straight line through P and C
Principal focus (F) Point where rays parallel to the axis converge (concave) or appear to diverge from (convex) after reflection
Focal length (f) Distance PF
Key relation: f = R/2 — the focus is midway between pole and centre of curvature.
Refraction of light — the bending of light as it passes from one transparent medium into another, caused by a change in its speed. Light bends towards the normal when going into a denser medium (slows down) and away from the normal when going into a rarer medium (speeds up).
Refractive index (n) — how much a medium slows light down: n = c/v (speed of light in vacuum ÷ speed in the medium). It has no unit. A higher n means optically denser.
Formulas
Sign convention (New Cartesian)
- The pole P is the origin. Distances are measured from P along the principal axis.
- Distances measured in the direction of incident light (rightward) are positive; against it (leftward) are negative.
- So the object distance u is always negative (object is in front of the mirror/lens).
- Heights above the principal axis are positive; below are negative.
Mirror formula and magnification
Quantity Formula
Mirror formula 1/v + 1/u = 1/f
Magnification m = h'/h = −v/u
Sign meanings for mirrors: concave mirror f is negative, convex mirror f is positive. A negative m means a real, inverted image; a positive m means a virtual, erect image. \ m\ 1 enlarged, < 1 diminished.
Refraction: laws and refractive index
Laws of refraction: (1) the incident ray, refracted ray, and normal lie in one plane; (2) Snell's law — the ratio sin i / sin r is constant for a given pair of media:
n₂₁ = sin i / sin r = v₁ / v₂ = n₂ / n₁
- Absolute refractive index: n = c/v (relative to vacuum).
- Speed of light in vacuum c = 3 × 10⁸ m/s.
- Example: for glass n = 1.5, so light travels at v = c/n = 2 × 10⁸ m/s in glass.
- In a rectangular glass slab, the emergent ray is parallel to the incident ray but laterally displaced — the bending towards the normal on entry is undone on exit.
Lens formula, magnification, power
Quantity Formula
Lens formula 1/v − 1/u = 1/f
Magnification m = h'/h = v/u
Power of a lens P = 1/f (f in metres); unit dioptre (D)
Sign meanings for lenses: convex (converging) lens f is positive, concave (diverging) lens f is negative. Convex lens power is positive, concave lens power negative. For lenses in contact, powers add: P = P₁ + P₂ + …
Image Formation Tables
Concave mirror
Object position Image position Nature & size
At infinity At F Real, inverted, highly diminished (point)
Beyond C Between F and C Real, inverted, diminished
At C At C Real, inverted, same size
Between C and F Beyond C Real, inverted, enlarged
At F At infinity Real, inverted, highly enlarged
Between P and F Behind the mirror Virtual, erect, enlarged
Uses: torches, headlights, shaving mirrors, dentists' mirrors, solar furnaces.
Convex mirror
A convex mirror always forms a virtual, erect, diminished image between P and F, for any object position. Wide field of view → used as rear-view mirrors and on blind corners.
Convex lens (mirrors ↔ lenses parallel)
Object position Image Nature
At infinity At F₂ Real, inverted, point-sized
Beyond 2F₁ Between F₂ and 2F₂ Real, inverted, diminished
At 2F₁ At 2F₂ Real, inverted, same size
Between F₁ and 2F₁ Beyond 2F₂ Real, inverted, enlarged
At F₁ At infinity Real, inverted, highly enlarged
Between F₁ and optical centre Same side as object Virtual, erect, enlarged
A concave lens always forms a virtual, erect, diminished image between F₁ and the optical centre.
Important Question Patterns
1. Numerical on mirror/lens formula (3 marks): given two of u, v, f, find the third and the magnification; state image nature. Always write the sign convention first — mark the object distance negative.
2. Power of a lens (1–2 marks): P = 1/f with f in metres; combined power of lenses in contact; convert dioptre ↔ focal length.
3. Ray diagram (2–3 marks): draw image formation for a stated object position; label at least two standard rays (parallel→through F, through F→parallel, through C/optical centre→undeviated).
4. Refractive index / speed (2 marks): n = c/v; compare optical densities; explain bending direction entering a denser/rarer medium.
5. Reasoning (2 marks): why convex mirror for rear-view; why concave mirror as a shaving/dentist mirror; lateral displacement through a glass slab; why a pencil looks bent in water.
⚡ Quick Revision
- f = R/2. Concave mirror & convex lens converge; convex mirror & concave lens diverge.
- Mirror formula 1/v + 1/u = 1/f, m = −v/u. Lens formula 1/v − 1/u = 1/f, m = v/u. (Note the sign difference between the two formulas.)
- Sign convention: measure from pole/optical centre; object distance u is always negative; +ve along incident light.
- m negative → real & inverted; m positive → virtual & erect. \ m\ 1 enlarged.
- Concave mirror f negative, convex mirror f positive; convex lens f (and power) positive, concave lens negative.
- Snell's law: sin i / sin r = n₂/n₁ = v₁/v₂ = constant. Denser medium → bend towards normal.
- n = c/v, c = 3×10⁸ m/s; higher n = optically denser (glass 1.5 water 1.33 air 1).
- Power P = 1/f (metres), unit dioptre (D); lenses in contact: powers add.
- Convex mirror image = always virtual, erect, diminished. Concave lens image = always virtual, erect, diminished.
- Glass slab: emergent ray parallel to incident ray, only laterally shifted.
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