List of top Physics Questions asked in CBSE CLASS XII

Find the ratio of energies of photons emitted during the transition of an electron in hydrogen atom from its:

(i) second permitted energy level to the first permitted energy level, and

(ii) the highest permitted energy level to the first permitted energy level.

An electron is revolving around a proton in a circular orbit of radius $r$. Using classical physics, prove that the ratio of value of the angular momentum and the value of magnetic moment of electron revolving in the orbit is a constant.

The radius of the innermost electron orbit of a hydrogen atom is $0.53\,\text{\AA}$. The radius of orbit in the second excited state is:

A ray of light passes from a medium of refractive index $\sqrt{2}$ to the medium of refractive index $1$. For the ray to just graze along the surface of separation, the angle of incidence should be:

(a) A long straight wire carries a steady current $I$ along the positive $y$-axis in a three-dimensional coordinate system. A particle of charge $+Q$ is moving with a velocity $\vec{v} = v_0\hat{i}$. Find the magnitude and direction of the magnetic force experienced by the particle at the instant it is at $(x, 0, 0)$.

(b) A galvanometer of resistance $100\,\Omega$ shows full scale deflection for a current of $5\,\text{mA}$. Explain how you will convert it into a voltmeter of range $(0 - 1\,\text{V})$. Obtain the value of the resistance required.

(a) The distance of an object from a concave mirror is increased from $12\,\text{cm}$ to $15\,\text{cm}$. It is observed that the new image becomes $\left(\dfrac{2}{5}\right)^{\text{th}}$ the size of the initial image. Calculate the focal length of the mirror.

(b) A light ray is incident on the horizontal upper face of a glass cube making an angle of $45^\circ$ with the face. While emerging, the ray just grazes the vertical face of the cube. Find the refractive index of the glass.

A thin convex lens with its two surfaces of radii of curvature $R_1$ and $R_2$ is made of a material of refractive index $\mu_2$. A medium of refractive index $\mu_1$ ($< \mu_2$) surrounds it. Using a ray diagram, derive the Lens Maker's formula when a point object placed on the principal axis in front of the surface of radius of curvature $R_1$ produces an image I on the other side of the lens.

(a) Find the expression for the induced charge on a surface of a linear dielectric slab (dielectric constant $K$) inserted between the parallel plates of a charged capacitor fully occupying the intervening region.

(b) Four capacitors, each of capacitance $1\,\mu\text{F}$ are to be connected to obtain a net capacitance of $0.75\,\mu\text{F}$. Draw a diagram to show the required combination.

In an interference pattern, $10$ fringes are obtained over a certain distance from its central maximum with light of wavelength $480\,\text{nm}$. The number of fringes obtained over the same distance with light of wavelength $600\,\text{nm}$ would be:

A $2.00\,\text{m}$ long wire, ABC made of steel ($\rho = 20 \times 10^{-8}\,\Omega\,\text{m}$ at $20^\circ\text{C}$) consists of two parts AB and BC, each of $1.00\,\text{m}$. Part AB has a radius of $0.70\,\text{mm}$ while BC has a radius of $0.35\,\text{mm}$. Find the magnitude of electric field at a point in part AB and part BC, at $20^\circ\text{C}$, if a steady current of $1.54\,\text{mA}$ passes through the wire.

The frequency of the incident light on a metal surface is doubled. Find the new value of the maximum kinetic energy of the electrons and comment on its magnitude in comparison to the initial maximum kinetic energy of electrons.

Calculate the distance of closest approach when an alpha particle of $4.50\,\text{MeV}$ approaches in head-on position, a nucleus ($Z = 79$), stops and reverses its direction.

The Balmer series of hydrogen atom lies in:

The focal lengths of the objective and the eyepiece of a compound microscope are $2\,\text{cm}$ and $3\,\text{cm}$ respectively. If they are kept $25\,\text{cm}$ apart, the tube length of the microscope is:

When radiation of suitable frequency is incident on a metal surface, electrons are emitted from the surface. This phenomenon is called photoelectric effect. Einstein explained this phenomenon considering the particle nature of radiation and using the law of conservation of energy. Wave theory of light could not explain this phenomenon.

A particle of mass m and charge q experiences a force $\vec{F}_e$ in an electric field $\vec{E}$ and it is directed along the field. When the particle is moving with velocity $\vec{v}$ in a magnetic field $\vec{B}$, it experiences a force $\vec{F}_B$, that is perpendicular to both $\vec{v}$ and $\vec{B}$.

In presence of both fields, the force acting on the particle is the vector sum of that due to $\vec{E}$ and that due to $\vec{B}$.

A current-carrying conductor also experiences a force in presence of a magnetic field. The force $d\vec{F}$ acting on a current-element $I\,d\vec{l}$ is perpendicular to both the current element, and the magnetic field $\vec{B}$. The net force acting on a conductor of a given length is the vector sum of all $d\vec{F}$s.

Differentiate between interference and diffraction of light.

Two coherent light waves, each of intensity $I_0$ produce interference pattern on a screen. Find the intensity at a point at which the path difference between them is (i) $\dfrac{\lambda}{3}$, and (ii) $\dfrac{\lambda}{4}$.

In a Young's double-slit experiment, fringes are obtained on a screen placed at a distance $D$ from the slits. It is observed that if the screen is moved $5\,\text{cm}$ towards the slits, the fringe width changes by $0.03\,\text{mm}$. If the distance between the slits is $1\,\text{mm}$, calculate the wavelength of the light used.

The resistance of a heating coil is $100\,\Omega$ at $25^\circ\text{C}$ and $120\,\Omega$ at $1025^\circ\text{C}$. (i) Find its resistance at $425^\circ\text{C}$. (ii) Draw a plot showing variation of its resistance as a function of increase in temperature.

An ac generator consists of a coil of 500 turns and size $25\,\text{cm} \times 100\,\text{cm}$. It is rotated at an angular speed of $60\,\text{rad/s}$ in a uniform magnetic field $B = 0.4\,\text{T}$ between two fixed pole pieces. If the resistance of the circuit, including that of the coil, is $250\,\Omega$, calculate:

(I) the maximum current drawn from the generator

(II) the flux through the coil when the current is zero

Give two examples of commercial generators. How is rotation of armature achieved in them? Explain briefly.

An object is placed at a distance of $15\,\text{cm}$ from a convex lens of focal length $10\,\text{cm}$. On the other side of the lens, a convex mirror is placed coaxially at a distance of $10\,\text{cm}$ from the lens. If the image formed by this combination coincides with the object itself, draw the ray diagram and find the focal length of the convex mirror.

A small bulb is placed at the bottom of a tank containing water to a depth of $1.00\,\text{m}$. Find the area of the surface of water through which light from the bulb can emerge from. The refractive index of water is $(4/3)$.

Write two disadvantages of transmitting ac over long distances at low voltage and high current.

Using a phasor diagram, derive an expression for the impedance of a series LCR circuit connected to an ac source. What is meant by resonance in this circuit? Obtain expression for the resonant frequency of this circuit.