List of top Questions asked in CBSE CLASS XII

An AC voltage \( V = 0.5 \sin (100\pi t) \) volt is applied, in turn, across a half-wave rectifier and a full-wave rectifier. The frequency of the output voltage across them respectively will be:
A thin converging lens of focal length 20 cm and a thin diverging lens of focal length 15 cm are placed coaxially in contact. The power of the combination is:
A thin convex lens \( L \) of focal length 10 cm and a concave mirror \( M \) of focal length 15 cm are placed coaxially 40 cm apart as shown in the figure. A beam of light coming parallel to the principal axis is incident on the lens. The final image will be formed at a distance of: 
A thin convex lens
Draw a ray diagram for the formation of the image of an object by a convex mirror. Hence, obtain the mirror equation.
Differentiate between a wavefront and a ray.
A dielectric slab of dielectric constant \( K \) and thickness \( t \) is inserted between plates of a parallel plate capacitor of plate separation \( d \) and plate area \( A \). Obtain an expression for its capacitance.
Using Gauss's law, show that the electric field \( E \) at a point due to a uniformly charged infinite plane sheet is given by \( E = \frac{\sigma}{2\epsilon_0} \), where symbols have their usual meanings.
Mention the factors on which the resonant frequency of a series LCR circuit depends. Plot a graph showing variation of impedance of a series LCR circuit with the frequency of the applied AC source.
With the help of a diagram, briefly explain the construction and working of an AC generator.
The focal length of an equiconvex lens:
A beam of light coming parallel to the principal axis of a convex lens \( L_1 \) of focal length 16 cm is incident on it. Another convex lens \( L_2 \) of focal length 12 cm is placed coaxially at a distance 40 cm from \( L_1 \). The nature and distance of the final image from \( L_2 \) will be:
At a given temperature, the number of intrinsic charge carriers in a semiconductor is \( 2.0 \times 10^{10} \) cm\(^{-3}\). It is doped with pentavalent impurity atoms. As a result, the number of holes in it becomes \( 8 \times 10^3 \) cm\(^{-3}\). The number of electrons in the semiconductor is:
Initially, during the formation of a p-n junction:
During the formation of a p-n junction:
When Ge is doped with a pentavalent impurity, the energy required to free the weakly bound electron from the dopant is about:
Two electric heaters have power ratings \( P_1 \) and \( P_2 \), at voltage \( V \). They are connected in series to a DC source of voltage \( V \). Find the power consumed by the combination. Will they consume the same power if connected in parallel across the same source?
Assertion (A): In interference and diffraction of light, light energy reduces in one region producing a dark fringe. It increases in another region and produces a bright fringe.
Reason (R): This happens because energy is not conserved in the phenomena of interference and diffraction.
A proton and an \( \alpha \)-particle are accelerated through different potentials \( V_1 \) and \( V_2 \) respectively so that they have the same de Broglie wavelengths. Find \( \frac{V_1}{V_2} \).
A ray of light is incident normally on one face of an equilateral glass prism of refractive index \( P \). When the prism is completely immersed in a transparent medium, it is observed that the emergent ray just grazes the adjacent face. Find the refractive index of the medium.
Assertion (A): The temperature coefficient of resistance is positive for metals and negative for p-type semiconductors.
Reason (R): The charge carriers in metals are negatively charged, whereas the majority charge carriers in p-type semiconductors are positively charged.
Assertion (A): When electrons drift in a conductor, it does not mean that all free electrons in the conductor are moving in the same direction.
Reason (R): The drift velocity is superposed over large random velocities of electrons.
Assertion (A): Electrons are ejected from the surface of zinc when it is irradiated by yellow light.
Reason (R): Energy associated with a photon of yellow light is more than the work function of zinc.
The phase difference between electric field \( \vec{E} \) and magnetic field \( \vec{B} \) in an electromagnetic wave propagating along the z-axis is:
In the Balmer series of the hydrogen atom, as the wavelength of spectral lines decreases, they appear:
A straight wire is kept horizontally along the east-west direction. If a steady current flows in the wire from east to west, the magnetic field at a point above the wire will point towards: