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Reflection and Refraction Basics

Part of the TNPSC Group 1 study roadmap. Science topic scienc-008 of Science.

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Reflection and Refraction Basics

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

  • Topic 8 in TNPSC Group 1 Science covers Light, Human Eye and the Electromagnetic Spectrum under General Studies Paper I (~3% weightage).
  • Mirror formula: 1/v + 1/u = 1/f (Cartesian sign convention; u, v, f in metres).
  • Lens formula: 1/v − 1/u = 1/f; Magnification m = hᵢ/hₒ = −v/u (dimensionless).
  • Snell’s law: n = sin i / sin r = c/v; Critical angle sin C = 1/n governs total internal reflection.
  • Vision defects: myopia → concave (diverging) lens; hypermetropia → convex (converging) lens.
  • EM spectrum order by increasing frequency: Radio < Micro < IR < Visible < UV < X-ray < Gamma; photon energy E = hν = hc/λ.
  • High-yield MCQs test sign-convention errors in the mirror formula, identification of corrective lenses, and EM-wave ordering.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Reflection and Refraction Basics

The law of reflection states that the angle of incidence equals the angle of reflection, with the incident ray, reflected ray and normal lying in the same plane. Refraction follows Snell’s law, n₁ sin i = n₂ sin r, where the refractive index n = c/v (ratio of light’s speed in vacuum to its speed in the medium).

Mirror and Lens Formulae

Concave mirrors and convex lenses converge light; convex mirrors and concave lenses diverge it. The two formulae look similar but differ in sign:

FeatureMirror formulaLens formula
Relation1/v + 1/u = 1/f1/v − 1/u = 1/f
f sign for concave/convergingNegativePositive
f sign for convex/divergingPositiveNegative
Power P (dioptre)—P = 1/f (f in metres)

A common trap: students apply the lens formula to a mirror (or vice versa) and silently carry the wrong sign for f.

Total Internal Reflection and Prisms

When light travels from a denser to a rarer medium, beyond the critical angle (sin C = 1/n) it reflects entirely back — this is total internal reflection (TIR). Optical fibres and mirages exploit TIR. A prism disperses white light into VIBGYOR because n varies with wavelength (red refracts least, violet most).

Human Eye and Vision Defects

The eye’s near point is ~25 cm and far point is infinity. Myopia (short-sight) has the image forming before the retina and is corrected with a concave lens. Hypermetropia (long-sight) has the image forming behind the retina and needs a convex lens.

Tip: Write the corrective lens as the shape of the lens, not as ”+” or ”−” power — the shape (concave/convex) is what MCQ options usually test.

Electromagnetic Spectrum

Waves are ordered by wavelength (decreasing) or frequency (increasing): Radio → Micro → IR → Visible → UV → X-ray → Gamma. Use E = hc/λ to compute photon energy, with c = 3 × 10⁸ m/s and h = 6.626 × 10⁻³⁴ J·s.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge Cases in Image Formation

A convex mirror always produces a virtual, erect, diminished image regardless of object position — useful as a rear-view mirror because it gives a wider field of view. For a concave mirror, the image nature flips when the object crosses f: beyond f the image is real and inverted; between f and the pole it becomes virtual and erect. Similarly, a convex lens gives a real, inverted image when u > 2f, but a virtual, enlarged, erect image when u < f (used in a magnifying glass). Missing these crossover points is the single most common reason students mis-classify images in TNPSC diagrams.

Sign Convention Pitfalls

The Cartesian sign convention treats the pole as origin, the principal axis as the x-axis, and the direction of incident light as positive. Hence for a real object u is always negative. For mirrors, f is negative for concave and positive for convex; for lenses the signs are reversed. A frequent examiner trap: give a numerical problem where the object is at u = −30 cm and the focal length is +20 cm, then ask the image distance. Plugging without sign awareness yields an absurd answer.

Dispersion and Atmospheric Optics

White light splits because n decreases with increasing wavelength (normal dispersion). The deviation produced by a prism is minimum at the angle of minimum deviation, where i = e — a numerical often quoted in paper I. Atmospheric refraction explains the sun being visible before actual sunrise and after sunset by about 2 minutes; scattering explains the red colour of sunsets (longer wavelengths survive after shorter blue wavelengths scatter out).

Connections and Common Mistakes

  • Health link: UV radiation damages the cornea (snow blindness); UV-B drives vitamin D synthesis in skin.
  • Communication link: Optical fibres carrying internet data rely on TIR — a syllabus cross-reference to “Science & Technology” topics.
  • Top mistakes: mixing up mirror vs lens sign of f, writing 3 × 10⁶ for c in E = hc/λ, and stating hypermetropia is corrected by a concave lens.

Practice Prompts

  1. An object is placed 30 cm in front of a concave mirror of focal length 15 cm. Find the image position, nature and magnification. (Expected: v = −30 cm, real, inverted, same size.)
  2. The refractive index of glass is 1.5. Calculate the critical angle for a glass-air interface. (Expected: sin C = 2/3 ⇒ C ≈ 41.8°.)

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