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Optics

Reflection, refraction, lenses, wave optics, diffraction

Key Concepts
  • Reflection: angle of incidence = angle of reflection
  • Snell's law: n₁ sinθ₁ = n₂ sinθ₂; TIR occurs when θ > critical angle
  • Lens formula and mirror formula use same sign convention
  • Young's double slit: interference pattern with bright and dark fringes
  • Diffraction: bending of waves around obstacles; single slit produces central maximum
Important Formulae
Mirror/lens formula 1/f = 1/v − 1/u
Magnification (mirror) m = −v/u
Snell's law n₁ sinθ₁ = n₂ sinθ₂
Critical angle sin C = 1/n (n = refractive index)
Fringe width (YDSE) β = λD/d
Lens maker's formula 1/f = (n−1)(1/R₁ − 1/R₂)
Quick Tips
  • New Cartesian sign convention: distances measured from pole; along incident ray = positive.
  • Power of lens P = 1/f (in metres); unit = dioptre. Lenses in contact: P = P₁ + P₂.
  • In YDSE: path difference = nλ for bright fringe; (2n−1)λ/2 for dark fringe.
Sample Practice Questions

  1. Refractive index of a medium = c/v. If light slows to 2×10⁸ m/s, n =

    • 1.0
    • 1.25
    • 1.5
    • 2.0
    Show answer

    Answer: 1.5

  2. Critical angle and total internal reflection occurs when light goes from:

    • Rarer to denser medium
    • Denser to rarer medium
    • Air to glass
    • Any medium to vacuum
    Show answer

    Answer: Denser to rarer medium

  3. In Young's double slit experiment, fringe width β = λD/d. If d is doubled:

    • β doubles
    • β halves
    • β stays same
    • β quadruples
    Show answer

    Answer: β halves

  4. Coherent sources are needed for:

    • Diffraction
    • Reflection
    • Refraction
    • Sustained interference pattern
    Show answer

    Answer: Sustained interference pattern

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Practice Questions

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