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.
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.
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.
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 |
Worked Examples
Example 1 — Mirror Formula (Concave Mirror)
Problem: An object is placed 30 cm in front of a concave mirror of focal length 15 cm. Find the image position and nature.
Solution:
Using sign convention: u = −30 cm, f = −15 cm (concave mirror, focus in front).
Apply mirror formula: 1/v + 1/u = 1/f 1/v + 1/(−30) = 1/(−15) 1/v = −1/15 + 1/30 = (−2 + 1)/30 = −1/30 v = −30 cm
Image is at 30 cm in front of mirror (same side as object).
Magnification m = −v/u = −(−30)/(−30) = −1
Nature: Real (v negative, same side), inverted (m negative), same size (|m| = 1).
Example 2 — Refraction and Snell's Law
Problem: A ray of light passes from air into glass (n = 1.5) at an angle of incidence 30°. Find the angle of refraction.
Solution:
n = sin i / sin r 1.5 = sin 30° / sin r = 0.5 / sin r sin r = 0.5 / 1.5 = 1/3 ≈ 0.333 r = sin⁻¹(0.333) ≈ 19.5°
The refracted ray bends toward the normal as light enters a denser medium.
Example 3 — Lens and Power
Problem: A convex lens has a focal length of 25 cm. What is its power?
Solution:
Convert f to metres: f = 0.25 m P = 1/f = 1/0.25 = +4 D
Positive power indicates a converging lens.
Common Mistakes
| Wrong Thinking | Correct Fix |
|---|---|
| Forgetting sign convention and using all positive values → wrong image position/nature. | Always apply New Cartesian sign convention; concave mirror f is negative, convex lens f is positive. |
| Confusing mirror formula (1/v + 1/u = 1/f) with lens formula (1/v − 1/u = 1/f). | Remember: "Mirror has a plus, Lens has a minus" in the formula. |
| Believing virtual images are always smaller; real images are always larger. | Size depends on object position relative to focus; virtual images can be magnified (e.g., magnifying glass). |
| Mixing up convex mirror and convex lens behaviour. | Convex mirror diverges light (virtual, diminished image); convex lens converges light (can form real or virtual image). |
| Ignoring units when calculating power → answer off by factor of 100. | Focal length must be in metres for power in dioptres. |
Quick Reference
- Concave mirror uses: shaving mirrors, dentist mirrors, headlights, solar furnaces.
- Convex mirror uses: rear-view mirrors (wider field of view).
- Convex lens uses: magnifying glass, camera, projector, human eye correction (hypermetropia).
- Concave lens uses: correction of myopia (short-sightedness).
- Total internal reflection: occurs only when light travels from denser to rarer medium and angle of incidence exceeds critical angle.
- Image by plane mirror: always virtual, erect, same size, laterally inverted, at same distance behind mirror as object is in front.