CBSE Class 12 Physics 2026 — Set 55/3/1

A telescope has an objective lens of focal length 144 cm and an eyepiece of focal length 6.0 cm. The magnifying power and the length of the telescope tube will be respectively : (A) 24,150 cm (B) 42,138 cm (C) 24,138 cm (D) 42,150 cm

In which of the following does total internal reflection NOT occur ? (A) Twinkling of stars (B) Brilliance of diamonds (C) Optical fibre (D) Reflecting prism

Radiation of wavelength 200 nm is incident on a photosensitive surface of work function 4.2 eV. The kinetic energy of the fastest photoelectrons emitted from this surface will be close to : (A) 3.5 eV (B) 3.0 eV (C) 2.5 eV (D) 2.0 eV

A proton and an alpha particle have equal momentum. The ratio of their kinetic energies ((Ep)/(E_α)) and the ratio of the de Broglie wavelengths associated with them ((λp)/(λ_α)) respectively are : (A) 2,1 (B) 1,2 (C) 4,1 (D) 1,4

If r₁ and r₂ are the radii of atomic nuclei of mass numbers 64 and 27 respectively, then the value of (r₁)/(r₂) is : (A) 1 (B) (4)/(3) (C) (3)/(4) (D) (27)/(64)

When the forward bias voltage in a semiconductor diode is changed from 0.8 V to 1.0 V, the forward current changes by 2.0 mA. The forward bias resistance of the diode will be : (A) 200Ω (B) 175Ω (C) 100Ω (D) 125Ω

The process named 'minority carrier injection' in a p-n junction diode occurs during : (A) forward biasing (B) reverse biasing (C) no biasing at low temperature (D) no biasing at high temperature

Assertion (A) : In a Wheatstone bridge circuit, if we interchange the position of the cell and the galvanometer, the balance condition (P)/(Q) = (R)/(S) remains unchanged. Reason (R) : (P)/(Q) = (R)/(S) ⇒ (Q)/(P) = (S)/(R), so the balance condition remains the same. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false.

Assertion (A) : The cylindrical soft iron core in a moving coil galvanometer only makes the magnetic field radial and does not affect the strength of the magnetic field. Reason (R) : In a moving coil galvanometer, the plane of the coil is always perpendicular to the magnetic field. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false.

Assertion (A) : Photoelectric current depends upon the intensity of the incident radiation. Reason (R) : Stopping potential is independent of the intensity of the incident radiation. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false.

Assertion (A) : Nuclear forces are always attractive. Reason (R) : The nuclear force between protons and neutrons in a nucleus is a weak force. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Both Assertion (A) and Reason (R) are false.

(a) In the given figure, a steady current I flows through the circuit when points A and C are connected by a wire of negligible resistance. Find the potential difference between points B and C. OR (b) A battery of emf 21 V and internal resistance 3Ω is connected to a resistor. If the current in the circuit is 3 A, find : (i) the resistance of the resistor. (ii) the terminal voltage of the battery.

Explain why diffraction of sound is more common in daily experience than that of light.

Explain the terms mass defect and binding energy. How are they related ?

A particle of mass M at rest splits up into two particles of masses m₁ and m₂ having non-zero velocities. Calculate the ratio of the de Broglie wavelengths associated with the two particles.

How are charge carriers created in an intrinsic semiconductor ? Explain.

(a) Establish the relation between drift velocity of electrons (vd) and electric current (I) in a conductor. (b) How is vd affected when the length of the conductor is doubled, keeping the voltage applied across the conductor constant ?

(a) A circular coil of 30 turns and radius 8.0 cm carrying a current of 6 A is suspended vertically in a uniform horizontal magnetic field of 1.0 T. The field lines make an angle of 30^° with the plane of the coil. Calculate the magnitude of the external torque that must be applied to prevent the coil from turning. What would happen if the circular coil is replaced by a planar coil of irregular shape that encloses the same area, keeping other parameters unchanged ? OR (b) An alpha particle (mass 6.4×10⁻²⁷ kg and charge 3.2×10⁻¹⁹ C) having 8.0 MeV energy enters a region of a uniform magnetic field of 0.5 T. If the field is directed perpendicular to the velocity of the particle, find the radius of the circular path described by the particle. Mention the condition under which the particle in this region (i) describes a helical path, and (ii) goes straight undeviated.

(a) Discuss the behaviour of an inductor connected to (i) a dc source, and (ii) a high frequency ac source. (b) What is the phase relation between current and voltage in an ideal inductor connected to an ac source ? Draw a phasor diagram for the circuit.

(a) Depict the variation of electric field (vecE) and magnetic field (vecB) with respect to the direction of propagation of an electromagnetic wave. Write their two important characteristics. (b) Show that dfrac1√(μ₀varepsilon₀) gives the velocity of an electromagnetic wave in free space.

A ray of light is travelling through a rectangular glass slab (refractive index (3)/(2)) and is incident on the horizontal glass-air surface at the critical angle for the two media. The slab is then brought in contact with water (refractive index (4)/(3)) such that a thin horizontal layer of water is formed on the surface of the slab. Find the angle at which the ray will emerge into air from the water-air surface.

Differentiate between nuclear fission and nuclear fusion. Give one example for each with nuclear reaction.

With the help of circuit diagrams, briefly explain the forward biasing and the reverse biasing of a p-n junction diode.

Capacitors are manufactured with certain standard capacitances and working voltages. However, these standard values may not be the ones actually needed. Two or more capacitors can be grouped in series or in parallel to achieve a desired capacitance and voltage. When connected in series, the total capacitance decreases while the voltage rating increases; in parallel, the total capacitance increases and the voltage rating stays the same. A capacitor stores energy in the electric field between its plates, and the stored energy is U = (1)/(2)CV². Two capacitors, one of 3muF and the other of 6muF, are connected in series in the circuit as shown in the figure, for a long time. (i) The total capacitance of the circuit is : (A) 6muF (B) 3muF (C) 9muF (D) 2muF (ii) The current in the 10Ω resistor is : (A) 0.3 A (B) 0.6 A (C) 0.2 A (D) 0 (iii) The potential difference between point A and B is : (A) 2 V (B) 0.3 V (C) 0.2 V (D) 3 V (iv)(a) The value of charge on the plates of the 6muF capacitor is : (A) 6muC (B) 4muC (C) 12muC (D) 8muC OR (iv)(b) The wire between two capacitors is cut at point P. The current in the circuit will : (A) increase (B) decrease (C) remain the same (D) first increase then become stable

A charged particle +q in an electric field vecE experiences a force in the direction of the field, and its kinetic energy changes. In a magnetic field vecB the moving charge also experiences a force, but this magnetic force is perpendicular to both the velocity vecv and vecB, so it cannot change the kinetic energy. Consider two charged particles 1 and 2 of masses m and (m)/(2) having charges -q and +2q respectively. They are accelerated from rest through the same potential difference V and acquire kinetic energies K₁ and K₂. They then enter a region of uniform magnetic field vecB perpendicular to their velocities. (i) The ratio of their kinetic energies (K₁)/(K₂) is : (A) (1)/(2) (B) (1)/(4) (C) 4 (D) 1 (ii) The ratio of the radii of the circular paths described by them (r₁)/(r₂) is : (A) (1)/(2) (B) √(2) (C) dfrac1√(2) (D) 2 (iii) Suppose particles 1 and 2 enter the magnetic field vecB = B₀hatk with velocities vecv₁ = v₁hati and vecv₂ = v₂hati. Then : (A) both particles revolve clockwise (B) both particles revolve anticlockwise (C) particle 1 revolves clockwise while particle 2 revolves anticlockwise (D) particle 1 revolves anticlockwise while particle 2 revolves clockwise (iv)(a) If the period of revolution for particle 1 is 4 s, then for particle 2 the period will be : (A) 1 s (B) 2 s (C) 4 s (D) 8 s OR (iv)(b) If the values of momentum for particles 1 and 2 are p₁ and p₂, then : (A) p₁ = (p₂)/(2) (B) p₁ = p₂ (C) p₁ = 2p₂ (D) p₁ = 4p₂

(a)(i) In the figure, OA and OB show the variation of electric potential V at a point due to two point charges Q₁ and Q₂ with (1)/(r) respectively, where r is the distance of the point from the charge. (I) Identify the nature of the two charges Q₁ and Q₂. (II) What is the value of (Q₁)/(Q₂) ? Justify your answer. (ii) Two point charges -2muC and 5muC are placed at (-30cm, 0) and (30cm, 0) respectively in an external electric field vecE = (A)/(x²)hati, where A = 9×10⁵Nm²C⁻¹. Find the electrostatic potential energy of this configuration. OR (b)(i) Two infinitely long straight wires having linear charge densities -λ and 3λ are held vertically parallel to each other, a distance r apart in free space. Find the nature and magnitude of the force per unit length exerted by one wire on the other. (ii) A small hollow conducting sphere of radius r₁ is given a charge Q. It is surrounded by a concentric conducting spherical shell of inner radius r₂ and outer radius r₃, carrying charge -3q. A point charge 2q is kept at the centre. Find : (I) the electric flux through a concentric spherical Gaussian surface of radius x for (1) x < r₁, and (2) r₁ < x < r₂; (II) the electric field at a point distant x from the centre for (1) x > r₃, and (2) r₁ < x < r₂; (III) the surface charge density on the inner surface of (1) the sphere, and (2) the shell.

(a)(i) A light bulb and an open coil inductor are connected in series across an ac source of variable frequency. How will the glow of the bulb be affected when : (I) an iron bar is inserted inside the coil, and (II) the frequency of the source is decreased ? Justify your answers (assume other factors remain unchanged). (ii) An ac voltage V = 280sin(100π t) volt is connected across a series LCR circuit in which R = 400Ω, L = (5)/(π) H and C = (50)/(π)muF. Taking √(2) = 1.4, calculate : (I) the impedance of the circuit, (II) the rms value of the current that flows in the circuit, (III) the power factor of the circuit. OR (b)(i) State Lenz's law and explain that it follows the law of conservation of energy. (ii) Write the dimensional formula for self-inductance. The current in a coil changes from 8.0 A to 2.0 A in 0.6 s. If the average emf induced in the coil is 50 V, calculate the self-inductance of the coil.

(a)(i) A point object is kept in front of a convex spherical surface of radius of curvature R. Draw the ray diagram to show the formation of the image and derive the relation between the object and image distances (u and v) in terms of the refractive index n of the medium and R. (ii) A convex lens of focal length 20 cm is used to form the image of an object placed 30 cm away from the lens. Find the position and nature of the image formed. OR (b)(i) Two thin converging lenses of focal lengths f₁ and f₂ are placed coaxially in contact. Derive an expression for the focal length of the combination. (ii) A beam of coherent light of wavelength 550 nm is incident normally on a pair of slits S₁ and S₂, each of width 1.2×10⁻⁶ m, separated by 1.1 mm. Fringes are observed on a screen 2.2 m away from the plane of the slits. Calculate : (I) the fringe width, (II) the distance of the second dark fringe from the central maximum, (III) what will happen when the entire apparatus is immersed in water.