Optics: images, interference and diffraction
Optics: images, interference and diffraction
NEB Class 12 Optics tests ray optics for mirrors and lenses, optical instruments, and wave optics including Young's double slit, diffraction and polarization. Expect sign-convention numericals, fringe-width calculations and conceptual comparisons between interference and diffraction.
Core rules
- Mirror/lens formula: 1/f = 1/v + 1/u for mirrors, and 1/f = 1/v − 1/u for lenses with the Cartesian sign convention; m = v/u (mirror) or m = v/u with signs.
- Power of a lens in dioptres: P = 1/f(m).
- Young's double slit: fringe width β = λD/d; bright fringes at path difference nλ, dark at (n + ½)λ.
- Single-slit diffraction minima: a sinθ = nλ; the central maximum is twice as wide as secondary maxima.
- Malus law for polarized light: I = I₀ cos²θ.
Worked example
In a Young's double-slit experiment, λ = 600 nm, slit separation d = 0.30 mm and screen distance D = 1.5 m. Find the fringe width. Convert: λ = 600 × 10⁻⁹ m, d = 0.30 × 10⁻³ m. β = λD/d = (600 × 10⁻⁹ × 1.5)/(0.30 × 10⁻³) = 3.0 × 10⁻³ m = 3.0 mm. Adjacent bright fringes are 3.0 mm apart; the pattern scales linearly with λ and D and inversely with d.
Traps
Using the wrong sign convention for virtual images; confusing fringe width with fringe position; entering slit separation in mm without converting; treating diffraction minima as maxima; assuming polarized light intensity is unchanged by an analyser; forgetting that coherent sources are required for stable interference.