Light – Reflection and Refraction
Mirror and lens formulas, magnification, and sign conventions.
Chapter Overview
Light follows laws of reflection and refraction. Study mirrors, lenses, ray diagrams, mirror/lens formulae, magnification, and power of lens — key numerical chapter for board exams.
Reflection
Law of reflection
∠i = ∠r; incident, reflected, normal in same plane
Mirror formula
1/f = 1/v + 1/u
Magnification
m = h'/h = −v/u
Refraction & Lenses
Snell's law
n₁ sin i = n₂ sin r
Lens formula
1/f = 1/v − 1/u (New Cartesian sign convention)
Power
P = 1/f (f in metres → P in dioptre D)
Refractive index
n = c/v = sin i / sin r
Sign Convention
- Object distance u negative
- Direction of incident light is positive x-axis
- Convex lens f positive; concave f negative
Solved Examples
Step-by-step solutions — read each step before checking the final answer.
Mirror formula
Concave mirror, u=−15 cm, f=−10 cm. Find v.
- 11/v = 1/f − 1/u = 1/(−10) − 1/(−15)
- 21/v = −0.1 + 0.0667 = −0.0333
- 3v ≈ −30 cm (real image)
Answer
v ≈ −30 cm
Lens power
Convex lens f = 0.25 m. Power?
- 1P = 1/f = 1/0.25
Answer
+4 D
Key Points to Remember
- ✓ Real image can be projected on screen
- ✓ Total internal reflection in optical fibres
- ✓ n for glass ≈ 1.5, water ≈ 1.33
Exam Tips
- • Draw ray diagrams for standard positions
- • Use sign convention consistently
- • Convert cm to m for power
Practice MCQs — Light – Reflection and Refraction
Test your understanding with topic-wise multiple choice questions. Explanations appear after each answer.
SI unit of power of lens: