Physics — Light

Overview

Light is a fundamental topic in the KAR TET Paper II Science section, bridging everyday observation with core physics principles. Questions typically test your understanding of how light behaves when it strikes mirrors and passes through lenses—reflection and refraction—and how images form in optical devices.

This topic connects directly to the upper-primary science curriculum (Classes 6–8) and often appears as 2–4 questions in the exam. Mastery requires you to visualize ray diagrams, apply mirror and lens formulas, and predict image characteristics (real/virtual, magnified/diminished, erect/inverted). Beyond the content, expect 1–2 pedagogy-linked questions on how to teach these concepts using experiments and demonstrations.

Students who score well here can quickly identify mirror/lens type from a problem, sketch the relevant ray diagram mentally, and apply sign conventions correctly.

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Key Concepts

• **Light travels in straight lines** (rectilinear propagation), which explains shadows, eclipses and pinhole cameras.

• **Reflection** is the bouncing back of light from a surface; it follows two laws: (i) angle of incidence = angle of reflection, (ii) incident ray, reflected ray and normal lie in the same plane.

• **Refraction** is the bending of light when it passes from one medium to another due to a change in speed; governed by Snell's law.

• **Refractive index (n)** = speed of light in vacuum / speed of light in medium; higher n means denser medium and more bending toward the normal.

• **Mirrors** are of two main types: **plane** (flat surface, virtual and same-size image) and **spherical** (concave or convex, curved surface).

• **Lenses** are transparent refracting devices: **convex (converging)** and **concave (diverging)**.

• **Real images** are formed by actual convergence of rays (can be caught on screen); **virtual images** are formed where rays appear to diverge from (cannot be caught on screen).

• **Sign convention (New Cartesian)**: distances measured from the optical centre/pole; distances in the direction of incident light are positive; heights above principal axis are positive.

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Formulas / Key Facts

| Concept | Formula / Fact | |---------|----------------| | Laws of Reflection | ∠i = ∠r; incident ray, reflected ray, normal in same plane | | Snell's Law | n₁ sin i = n₂ sin r (or n = sin i / sin r for air-to-medium) | | Mirror Formula | 1/v + 1/u = 1/f | | Lens Formula | 1/v − 1/u = 1/f | | Magnification (mirror) | m = −v/u = h'/h | | Magnification (lens) | m = v/u = h'/h | | Power of Lens | P = 1/f (f in metres); unit is dioptre (D) | | Relation: R and f | f = R/2 (for spherical mirrors) | | Critical Angle | sin C = 1/n (total internal reflection occurs when i > C, light going from denser to rarer) | | Speed of light in vacuum | ≈ 3 × 10⁸ m/s |