Schooling · Class 10 · Science · Light – Reflection and Refraction · Chapter on a page
CBSE Class 10 · Science · NCERT Science
Light – Reflection and Refraction: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 26 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
This note's current text has no applied fact-check on record.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Light travels in straight lines in a…
- The law of reflection states
- Spherical mirrors have a principal…
- A concave mirror can form real…
- Refraction occurs because light…
- Snell's law
- A convex lens converges light and…
- Formulas and facts
- 1/v + 1/u = 1/f
- m = h'/h = −v/u
- Relation between focal length and…
- 1/v − 1/u = 1/f
- m = h'/h = v/u
- P = 1/f (f in metres)
- Refractive index of medium 2 with…
- Watch out for
- Forgetting sign conventions and…
- Confusing focal length with radius…
- Thinking convex mirrors can form…
- Using the mirror formula for a lens…
- Believing light bends away from…
What this chapter is about
This chapter introduces how light behaves when it strikes surfaces and when it passes from one medium to another. You will learn the laws that govern reflection from mirrors and refraction through lenses, and how images are formed in each case.
Key ideas
- Light travels in straight lines in a uniform medium; this is called rectilinear propagation.
- The law of reflection states: the angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal all lie in the same plane.
- Spherical mirrors have a principal axis, centre of curvature (C), pole (P) and focus (F); for small apertures, the focal length f = R/2, where R is the radius of curvature.
- A concave mirror can form real, inverted images or virtual, erect images depending on the object's position; a convex mirror always forms a virtual, erect, diminished image.
- Refraction occurs because light changes speed when entering a different medium; the refractive index n = speed of light in vacuum / speed of light in the medium.
- Snell's law: n₁ sin i = n₂ sin r, where i is the angle of incidence and r is the angle of refraction.
- A convex lens converges light and can form real or virtual images; a concave lens diverges light and always forms a virtual, erect, diminished image.
- The sign convention (New Cartesian): distances measured in the direction of incident light are positive; heights above the principal axis are positive.
Formulas and facts to remember
- Mirror formula: 1/v + 1/u = 1/f, where u = object distance, v = image distance, f = focal length.
- Magnification (mirror): m = h'/h = −v/u, where h' is image height and h is object height.
- Relation between focal length and radius of curvature: f = R/2.
- Lens formula: 1/v − 1/u = 1/f.
- Magnification (lens): m = h'/h = v/u.
- Power of a lens: P = 1/f (f in metres), unit is dioptre (D).
- Refractive index of medium 2 with respect to medium 1: n₂₁ = sin i / sin r = v₁ / v₂.
- Absolute refractive index of glass ≈ 1.5; of water ≈ 1.33; of air ≈ 1.0003.
Common mistakes
- Forgetting sign conventions and putting all distances as positive → always assign signs first based on New Cartesian convention before substituting.
- Confusing focal length with radius of curvature → remember f = R/2, not f = R.
- Thinking convex mirrors can form real images → convex mirrors always form virtual, erect, diminished images.
- Using the mirror formula for a lens problem → mirror formula is 1/v + 1/u = 1/f; lens formula is 1/v − 1/u = 1/f.
- Believing light bends away from normal when entering a denser medium → light bends towards the normal when going from rarer to denser medium.
Quick revision
- Angle of incidence = angle of reflection; both rays and normal lie in one plane.
- Concave mirrors converge light; convex mirrors diverge light.
- Mirror formula: 1/v + 1/u = 1/f; lens formula: 1/v − 1/u = 1/f.
- Refractive index n = c / v; higher n means slower light and more bending.
- Power of lens in dioptres = 1 / focal length in metres.
- Use sign convention consistently: measure from the optical centre or pole along the principal axis.