PHOTOELECTRIC EFFECT:
  Light knocks electrons from metal surface
  Conditions: frequency of light must exceed threshold frequency (f₀)
  Threshold frequency: f₀ = φ/h (φ = work function of metal)
  Maximum KE of ejected electron: KE_max = hf - φ = e × stopping potential
  
  Key observations:
    KE depends on frequency, NOT intensity
    Number of electrons depends on intensity
    Effect is instantaneous (no time delay)
    Below threshold: no emission regardless of intensity

DE BROGLIE WAVELENGTH:
  Particle behaves as wave: λ = h/p = h/mv
  For electron accelerated through potential V:
    λ = h/√(2meV) ≈ 1.227/√V nm (for electron)

BOHR'S MODEL OF HYDROGEN ATOM:
  rn = 0.529 × n²/Z Å (radius of nth orbit)
  En = -13.6 × Z²/n² eV (energy of nth level)
  For hydrogen (Z=1): E₁=-13.6, E₂=-3.4, E₃=-1.51, E₄=-0.85 eV
  
  Spectral series:
    Lyman (n→1): UV region
    Balmer (n→2): visible region | Hα (red) 656nm, Hβ (blue-green), Hγ (violet)
    Paschen (n→3): infrared region

NUCLEAR PHYSICS:
  Binding energy: BE = (Zm_p + Nm_n - M_nucleus) × 931.5 MeV/u
  Mass defect: Δm = Zm_p + Nm_n - M_nucleus
  
  Radioactive decay: N = N₀e^(-λt) = N₀(½)^(t/T½)
    T½ = 0.693/λ | Mean life τ = 1/λ = 1.44 T½
  
  Alpha decay: A decreases by 4, Z decreases by 2
  Beta decay: A unchanged, Z increases by 1 (β⁻) or decreases by 1 (β⁺)
  Gamma: no change in A or Z (just energy emission)
  
  Nuclear fission: heavy nucleus splits (U-235 + neutron)
  Nuclear fusion: light nuclei combine (H + H → He, releases more energy/kg)

REFRACTION:
  Snell's law: n₁sinθ₁ = n₂sinθ₂
  Total internal reflection: when light goes from denser to rarer and θ > critical angle
  Critical angle: sin θc = n₂/n₁ (n₁ > n₂)
  Applications: optical fibre (TIR), mirage, diamond sparkle

LENSES:
  Lens formula: 1/v - 1/u = 1/f
  Sign convention: incident light direction = +x; distances from optical centre
  Power: P = 1/f(metres) in diopters
  Combination: P_total = P₁ + P₂ + P₃
  Magnification: m = v/u = -image size/object size

WAVE OPTICS:
  Young's double slit:
    Path difference: Δ = dsinθ ≈ dy/D
    Constructive: Δ = nλ | Destructive: Δ = (n+½)λ
    Fringe width: β = λD/d
  
  Diffraction: bending of light around edges
    Single slit first minimum: a sinθ = λ