Instruments: Vernier LC = 0.02 mm; Screw gauge LC = 0.01 mm. Zero error: Positive → Subtract, Negative → Add (PS-NA). Backlash error in screw gauge: rotate one direction only.

Sound & Pendulum: v = 2f(l_{2} − l_{1}) eliminates end correction (preferred over 4fl_{1}). $T^{2}$ vs l slope = 4π^{2}/g; effective length = string + bob radius.

Electrical & Bridge: Metre bridge: R/S = l/(100 − l); interchange → l' = 100 − l. Resistivity ρ = RA/L; area A = π$d^{2}$/4.

Optics: Concave mirror and lens f < 0; convex mirror and lens f > 0. Mirror: 1/v + 1/u = 1/f. Lens: 1/v − 1/u = 1/f. Prism minimum deviation: i = e.

Semiconductors: Si V_th = 0.7 V; Ge V_th = 0.3 V ("Silicon Seven, Germanium Three"). Zener diode: reverse breakdown at V_Z → constant voltage regulator. LED emits light in forward bias (V_th ≈ 1.5–3 V).