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Optics & Modern Physics

Ray optics, wave optics, photoelectric effect, atomic models, and nuclear physics

Key Concepts
  • Snell's law governs refraction; total internal reflection occurs beyond the critical angle
  • Mirror and lens formula share the same sign convention
  • Young's double slit produces interference fringes
  • Photoelectric effect: electrons are ejected only above the threshold frequency
  • Bohr model: energy is quantised as E_n = −13.6/n² eV
Important Formulae
Mirror / lens formula 1/f = 1/v − 1/u
Snell's law n₁ sinθ₁ = n₂ sinθ₂
Lens maker's formula 1/f = (n−1)(1/R₁ − 1/R₂)
Fringe width (YDSE) β = λD/d
Photoelectric effect KE_max = hf − φ
Radioactive half-life t₁/₂ = 0.693/λ
Quick Tips
  • Power of a lens P = 1/f (in metres), measured in dioptres; lenses in contact add.
  • Bright fringe when path difference = nλ; dark when (2n−1)λ/2.
  • Mass defect × c² gives the nuclear binding energy.
Sample Practice Questions

  1. The power of a lens of focal length 0.5 m is:

    • 0.5 D
    • 1 D
    • 2 D
    • 5 D
    Show answer

    Answer: 2 D

  2. The energy of a photon of light of wavelength λ is given by:

    • hλ
    • hc/λ
    • h/λ
    • c/λ
    Show answer

    Answer: hc/λ

  3. The lens formula is:

    • 1/f = 1/v + 1/u
    • 1/f = 1/v − 1/u
    • f = v − u
    • 1/f = u − v
    Show answer

    Answer: 1/f = 1/v − 1/u

  4. Total internal reflection can occur only when light travels from a:

    • Rarer to a denser medium
    • Denser to a rarer medium
    • Vacuum to glass
    • Medium to itself
    Show answer

    Answer: Denser to a rarer medium

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Practice Questions

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