List of top Questions asked in CBSE CLASS XII

Define coherent sources. Can two independent sodium lamps be coherent?
State Huygens' principle. Plane wave incident at angle i on reflecting surface. Construct reflected wavefront and prove i=r.
A ray of light is incident normally on a refracting face of a prism of prism angle A and suffers a deviation of angle δ. Prove that the refractive index n of the material of the prism is given by:n = sin(A+δ)/sin A
A ray of light passes through a triangular prism. Show graphically how the angle of deviation varies with the angle of incidence. Hence define the angle of minimum deviation
(i) Draw a step-up transformer diagram. Describe its working principle. Explain 3 energy loss causes.
(ii) Does a step-up transformer violate energy conservation? Explain.
(iii) A step-up transformer (N_p=200, N_s=3000) has V_p=90V, I_s=2A. Calculate V_s and I_p.
(i) Derive the impedance of an R-C series circuit connected to an AC source v = v_m sin(ωt).
(ii) When does an inductor act as a conductor? Explain.
(iii) A lamp (R) and inductor (L) in series with 220V-50Hz AC give 11A. With 110V DC, it gives 11A. Find L.
(a) "EM wavelength correlates with the size of the radiating system." Give two examples.
(i) Why do long-distance radio broadcasts use shortwave bands?
(ii) Why are X-ray telescopes satellite-based, while optical/radio telescopes are ground-based?
(a) State Bohr's postulates for the hydrogen atom.
(b) A hydrogen atom de-excites from level n to (n-1). Show that emitted frequency ν ∝ α/n³ for large n (α = constant).
An air-filled parallel plate capacitor (d = 1mm, C = 20pF) is charged to 4.0µC. Find work done to increase plate separation to 5mm (constant charge).
Differentiate between diffusion and drift current. Explain their role in p-n junction formation.
Who first produced and observed EM waves in the 25mm-5mm range?
A transformer has 500 primary turns and 5000 secondary turns. Input is 220V-50Hz. What's the output?
(a) Two characteristic properties of nuclear force. (b) Plot potential energy of a nucleon pair vs. separation. Two conclusions from the plot.
Coils P and Q. Current in P induces current in Q. (a) Explain the phenomenon. (b) Two factors affecting current in Q. (c) Direction of current in Q when current is in P and (i) R increases, (ii) R decreases.
A bar magnet (2.5 J/T) is aligned with a 0.32 T field. (a) Find work done to align it (i) normal, (ii) opposite to the field. (b) Find torque in both cases.
(a) Define current density. Scalar or vector? E field in a conductor (n electrons/volume, mass m, charge -e, relaxation time τ). Show j = αE, where α = ne²/mτ. (b) What's a Wheatstone bridge? Balance conditions?
Charges Q1, Q2, and Q3 are at (-d,0), (0,0), and (d,0). Q1=Q3, Q2>0. Find the nature and value of Q1 for zero potential energy.
How does conductivity of an intrinsic semiconductor vary with temperature? Show in a plot.
An object is placed 30 cm in front of a concave mirror (radius of curvature 40 cm). Find: (i) image position (ii) magnification
Two batteries (6V, 0.8Ω and 3V, 0.2Ω) are connected in series with external resistance R. Find R so that the potential difference across the 6V battery is zero.
An electron and a proton have the same wavelength λ. This implies they have the same:
Two beams (3.3 eV and 11.3 eV) illuminate a metal (work function 2.3 eV). Find the ratio of maximum electron speeds.
Electric and magnetic fields of EM waves are:
A conducting circular loop (radius shrinking at 1 mm/s) is in a uniform magnetic field B = 50 mT, plane perpendicular to the field. Find the induced emf when the radius is 4 cm.
A wire bent into circular loop A carries current I. The same wire is bent into loop B with two turns, same current. Find the ratio of magnetic fields at the centers of A to B.