Detailed, exam-oriented notes covering every syllabus unit.
Aim: to determine the wavelength of a monochromatic source using the circular interference fringes formed between a plano-convex lens and a flat glass plate.
Theory: for the reflected system, the diameter of the nth dark ring satisfies Dn² = 4nλR, so a plot of Dn² against ring number n gives a straight line whose slope yields λ once the radius of curvature R of the lens is known (or vice versa).
Fig. L1.1 (Animated) — Newton's rings form as concentric dark and bright circles around the point of contact between the plano-convex lens and the flat plate; each successive ring (shown here forming outward in sequence) satisfies Dₙ² = 4nλR for the n-th dark ring.
Procedure highlights: align the travelling microscope on the ring system illuminated by a sodium lamp through a 45° glass plate; measure diameters of several dark rings on both sides of the centre for accuracy; plot Dn² vs n and compute λ from the slope.
Common errors: non-coincidence of the lens/plate contact point with true zero order, parallax while reading the microscope scale, and dust between the surfaces distorting ring shape.
Aim: to determine the wavelength of sodium light using the interference fringes produced by a biprism.
Theory: two virtual coherent sources S₁, S₂ are created by refraction through the two halves of a thin-angle biprism. Fringe width β = λD/d, where d is the separation of the virtual sources (found by the displacement method using a convex lens) and D is the source-to-eyepiece distance.
Key measurement: d is obtained indirectly — a convex lens is placed between the biprism and the eyepiece and moved to two conjugate positions where sharp images of S₁ and S₂ form; d = √(d₁d₂).
Aim: to determine the wavelength of spectral lines using a plane transmission grating on a spectrometer.
Theory: principal maxima satisfy (a+b) sinθ = nλ. The grating element (a+b) is known from the number of lines/cm engraved on the grating; measuring the angle of diffraction θ for a known order n gives λ.
Procedure: the spectrometer must first be adjusted for parallel rays (Schuster's method) and the grating set for normal incidence before readings of the diffracted spectral lines are taken on both sides of the direct image.
Aim: to determine the specific resistance (resistivity) of the material of a given wire.
Theory: Carey Foster's bridge is a modified metre-bridge used for comparing two nearly equal resistances with high precision by taking a balance-point shift when the two resistance arms are interchanged, eliminating end-corrections. Resistivity ρ = RA/L, where R is resistance, A the cross-sectional area, and L the length of the wire.
Aim: to study the Hall effect in a semiconductor sample and determine the Hall coefficient, carrier concentration and mobility.
Theory: when a current-carrying semiconductor is placed in a transverse magnetic field, a transverse (Hall) voltage VH develops due to the Lorentz force on charge carriers. Hall coefficient RH = VHt/(IB); carrier density n = 1/(RHe); mobility μ = σRH, where σ is the material's conductivity.
Significance: the sign of VH identifies whether the semiconductor is n-type or p-type — one of the classic tests of majority carrier type.
Aim: to determine the energy band gap Eg of a semiconductor from the temperature dependence of its reverse saturation current or resistance.
Theory: for an intrinsic/near-intrinsic semiconductor, resistance follows R = R₀ exp(Eg/2kBT). A plot of ln R against 1/T gives a straight line of slope Eg/2kB, from which the band gap is extracted.
Aim: to trace the B–H (hysteresis) loop of a given ferromagnetic specimen and determine susceptibility, permeability, retentivity and coercivity.
Theory: as the applied magnetic field H is cycled, the induced flux density B lags behind H, tracing a closed loop whose area represents the energy dissipated per cycle as hysteresis loss — directly relevant to selecting core materials for transformers and inductors.
Fig. L1.6 (Animated) — The red point traces one full cycle of the B–H hysteresis loop of the ferromagnetic sample; the loop's area (traced repeatedly here) represents the energy dissipated as heat per unit volume per cycle of magnetisation.
Aim: to determine the coefficient of viscosity of a given liquid, typically by Poiseuille's method or the terminal-velocity (Stokes') method with a falling sphere.
Theory (Stokes' method): a small sphere falling through a viscous liquid reaches terminal velocity v when viscous drag balances the net gravitational force: η = 2r²g(ρ−σ)/9v, where r is the sphere radius, ρ its density and σ the liquid's density.
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