Last Updated: April 2026
Chapter 10 — Light: Reflection and Refraction is one of the highest-scoring chapters in CBSE Class 10 Science. In CBSE Board 2025, this chapter contributed 8–10 marks including numerical questions (3 marks) and conceptual questions. The chapter has a 100% appearance rate in board papers for the past 10 years. This complete guide covers every concept, diagram, formula, and 10 practice MCQs.
Laws of Reflection
When light falls on a surface and bounces back, it obeys two laws:
- The angle of incidence (∠i) equals the angle of reflection (∠r): ∠i = ∠r
- The incident ray, reflected ray, and normal all lie in the same plane.
Spherical Mirrors
Concave Mirror
A mirror with the reflecting surface curved inward (like the inside of a bowl).
- Used as: shaving/makeup mirror (object within F → virtual, magnified); dentist’s mirror; headlight reflectors; solar furnaces
- Forms real, inverted images (when object beyond F); virtual, erect, magnified (when object between P and F)
Convex Mirror
A mirror with the reflecting surface curved outward.
- Always forms virtual, erect, diminished images
- Used as: rear-view (wing) mirrors in vehicles — wide field of view
Mirror Formula and Magnification
Mirror formula: 1/f = 1/v + 1/u
Where: f = focal length, v = image distance, u = object distance (all measured from pole P)
Sign Convention (New Cartesian):
- Distances measured from pole P
- Distances in the direction of incident light = POSITIVE
- Distances opposite to incident light = NEGATIVE
- Heights above principal axis = POSITIVE; below = NEGATIVE
- For concave mirror: f is negative (f = −R/2)
- For convex mirror: f is positive
Magnification: m = h’/h = −v/u
- m > 0: image is virtual and erect
- m < 0: image is real and inverted
- |m| > 1: image is magnified; |m| < 1: image is diminished
Image Formation by Concave Mirror — Ray Diagram Table
| Object Position | Image Position | Nature | Size |
|---|---|---|---|
| At infinity (∞) | At F (focus) | Real, inverted | Point size |
| 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 | Magnified |
| At F | At infinity | Real, inverted | Highly magnified |
| Between F and P | Behind mirror | Virtual, erect | Magnified |
Refraction of Light
Refraction is the bending of light when it passes from one medium to another due to a change in speed.
Laws of Refraction (Snell’s Law)
- The incident ray, refracted ray, and normal are all in the same plane.
- n₁ sin θ₁ = n₂ sin θ₂ (Snell’s Law)
Refractive Index
n = c/v (ratio of speed of light in vacuum to speed in medium)
n = sin θᵢ / sin θᵣ (when light goes from less dense to more dense medium)
| Medium | Refractive Index (n) |
|---|---|
| Air/Vacuum | 1.0 |
| Water | 1.33 |
| Glass | 1.5 |
| Diamond | 2.42 |
Higher n = light bends more = optically denser medium
Lenses
Convex (Converging) Lens
- Thicker at centre, thinner at edges
- Focal length: POSITIVE
- Uses: reading glasses (hypermetropia), microscopes, cameras, projectors
Concave (Diverging) Lens
- Thinner at centre, thicker at edges
- Focal length: NEGATIVE
- Uses: spectacles for myopia (nearsightedness), peepholes
Lens Formula and Magnification
Lens formula: 1/f = 1/v − 1/u
Magnification: m = h’/h = v/u
Power of lens: P = 1/f (where f is in metres, P is in Diopters — D)
- Convex lens: P is positive
- Concave lens: P is negative
- P = +2 D means f = 0.5 m = 50 cm
Image Formation by Convex Lens
| Object Position | Image Position | Nature | Size |
|---|---|---|---|
| At ∞ | At F₂ | Real, inverted | Point size |
| 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 | Magnified |
| At F₁ | At ∞ | Real, inverted | ∞ size |
| Between O and F₁ | Same side as object | Virtual, erect | Magnified |
Important Numerical Problems
Problem 1: An object is placed 15 cm in front of a concave mirror of focal length 10 cm. Find image distance.
Solution: 1/f = 1/v + 1/u; u = −15 cm, f = −10 cm
1/v = 1/f − 1/u = 1/(−10) − 1/(−15) = −1/10 + 1/15 = (−3+2)/30 = −1/30
v = −30 cm (real, inverted image 30 cm in front of mirror)
Problem 2: A lens has focal length 25 cm. Find power.
P = 1/f (in m) = 1/0.25 = +4 D
Practice MCQs — CBSE Class 10 Chapter 10
Practice Quiz — 10 CLAT-Style Questions
Click an option to reveal the answer and explanation.
Common Board Exam Questions
- State and explain the laws of reflection. (2 marks)
- An object is placed 20 cm in front of a convex mirror of focal length 10 cm. Find image distance and magnification. (3 marks)
- Draw a ray diagram for concave mirror when object is placed between focus and centre of curvature. (3 marks)
- Explain why a convex mirror is preferred as a rear-view mirror. (2 marks)
- A person uses spectacles of power −3.5 D. Find focal length and state the defect of vision. (3 marks)
Frequently Asked Questions (FAQ)
What is the difference between concave and convex lenses?
A concave (diverging) lens is thinner at the centre and thicker at the edges — it diverges light rays and has a negative focal length. Used for myopia (short-sightedness). A convex (converging) lens is thicker at the centre and thinner at edges — it converges light rays and has a positive focal length. Used for hypermetropia (long-sightedness). Remember: Concave = Cave-like = sunken = diverging.
How do I remember the mirror formula vs lens formula?
Mirror formula: 1/f = 1/v + 1/u (both terms add on right side). Lens formula: 1/f = 1/v − 1/u (subtraction on right side). A mnemonic: “Lens has a minus, mirror has a plus.” Always apply sign convention carefully: for mirrors, object distance u is negative (object in front); for lenses, the same applies. All focal lengths and distances use the New Cartesian sign convention.
Why does a stick appear bent when placed in water?
When light travels from water (denser, n=1.33) to air (rarer, n=1.0), it bends away from the normal (angle of refraction > angle of incidence). Our eyes perceive light as travelling in straight lines, so the stick appears to be at the position from which the refracted rays appear to come — above the actual position. This is why pools appear shallower than they are and the stick appears bent at the water surface.
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