List of top Physics Questions asked in CBSE CLASS XII

An astronomical telescope consists of two converging lenses. One of them of large aperture and large focal length is called objective lens and the other one, of smaller focal length and smaller aperture is called the eyepiece. It is used to see distant objects which are not seen clearly with naked eyes. The image formed by the objective lens acts as an object for the eyepiece and the final image produced by the eyepiece is magnified.

A student sets up the circuit as shown in the figure to find the value of unknown resistance X and records a set of readings of the voltmeter and the ammeter by using the rheostat.

The assertion that V = IR is a statement of Ohm’s law is not true. Why ?

A low voltage supply from which one needs high currents must have very low internal resistance. Why ?

The electron drift speed is estimated to be only a few mm/s for currents in the range of a few amperes. How, then, is the current established almost the instant a circuit is closed ?

An equipotential surface through a point is normal to the electric field at that point.Explain

Obtain an expression for the work done to dissociate the system of three charges \(q\), \(-4q\) and \(2q\) placed at the vertices A, B and C respectively of an equilateral triangle of side \(a\).

An inductor of \(\frac{5}{\pi}\,H\), a capacitor of \(\frac{50}{\pi}\,\mu F\) and a resistor of \(400\,\Omega\) are connected in series across an ac voltage \[ v = 140 \sin(100\pi t)\,V. \] Calculate:
• (I) impedance of the circuit
• (II) rms value of current in the circuit} (Take \(\sqrt{2}=1.4\))

When a dielectric is placed in an external electric field, the electric field inside the dielectric is less than that outside it.Explain

The number of turns in the primary and the secondary coil of an ideal transformer are 100 and 5000 respectively. If 3.3 kW power is supplied to the transformer at 220 V, find:
• (I) current in the primary coil
• (II) output voltage

Explain the potential difference between the plates of a charged parallel plate capacitor decreases when its plates are brought closer.

Draw a labelled diagram of a step-up transformer. Obtain the ratio of secondary voltage to primary voltage in terms of number of turns in the two coils.

A ray of light QP is incident normally on the face BC of a triangular prism ABC of refractive index 1.5 kept in air, as shown in the figure. Trace the path of the ray as it passes through the prism and give relevant explanation. [Assume $\angle B = 60^\circ, \angle C = 30^\circ, \angle A = 90^\circ$ and face BC is the hypotenuse].

Define the terms (I) resonant frequency, and (II) power factor of a series LCR circuit. For what value of the power factor will the power dissipated in the circuit be maximum?

A plane wave is incident on a reflecting surface. Using Huygens principle, show how it is reflected from the surface. Hence, verify the law of reflection.

Depict refraction of a plane wave by a convex lens.

What is the difference between a ray and a wavefront?

Derive the relation for the refractive index ($\mu$) of a prism in terms of angle of minimum deviation ($\delta_m$) and angle of prism ($A$).

Define refractive index of a medium in terms of speed of light.

With the help of a circuit diagram, explain the working of a full wave rectifier. Depict the input and output waveforms.

Radiation of wavelength \(\lambda\) is incident on a photosensitive surface. Find the de Broglie wavelength of electrons emitted from the surface. Assume that the work function of the surface is negligible.

In the nuclear reaction : \[ {}_{1}^{2}\text{H} + {}_{1}^{2}\text{H} \rightarrow {}_{2}^{\text{A}}\text{X} + {}_{0}^{1}\text{n} \] (a) Find the value of A.
(b) Calculate the amount of energy released in the reaction.
Given : \( m\left({}_{1}^{2}\text{H}\right) = 2.014102\text{ u} \), \( m\left({}_{2}^{\text{A}}\text{X}\right) = 3.016049\text{ u} \), \( m\left({}_{0}^{1}\text{n}\right) = 1.008665\text{ u} \), \( 1\text{ u} = 931.5\text{ MeV}/c^2 \)

What are infrared waves ? Why are these waves referred to as ‘heat waves’ ? Give any two uses of these waves.

Explain how the dual aspect of matter is evident in the de Broglie relation.