List of top Physics Questions asked in CBSE CLASS XII

The ratio of the intensities at maxima to minima in Young’s double-slit experiment is \( 25 : 9 \). Calculate the ratio of intensities of the interfering waves.
The frequency of a photon of energy 1.326eV is:
The mass numbers of two nuclei A and B are 27 and 64 respectively. The ratio of their radii \( \left(\frac{r_A}{r_B}\right) \) will be:
(a) Briefly explain Einstein’s photoelectric equation.
(b) Four metals with their work functions are listed below:
K = 2.3 eV, Na = 2.75 eV, Mo = 4.17 eV, Ni = 5.15 eV.
The radiation of wavelength 330 nm from a laser source placed 1 m away, falls on these metals.
Which of these metals will not show photoelectric emission?
What will happen if the laser source is brought closer to a distance of 50 cm?
A galvanometer of resistance \( G \) is converted into a voltmeter to measure up to \( V \) volts by connecting a resistance \( R_1 \) in series with the coil. If \( R_1 \) is replaced by \( R_2 \), then it can only measure up to \( \frac{V}{2} \) volts. Find the value of the resistance \( R_3 \) (in terms of \( R_1 \) and \( R_2 \)) needed to convert it into a voltmeter that can read up to 2 V.
Define current sensitivity of a galvanometer. “Increasing the current sensitivity may not necessarily increase the voltage sensitivity.” Give reason.
Explain with the help of a labelled ray diagram the formation of final image by an astronomical telescope at infinity. Write the expression for its magnifying power.
The total magnification produced by a compound microscope is 20. The magnification produced by the eyepiece is 5. When the microscope is focused on a certain object, the distance between the objective and eyepiece is observed to be 14 cm. Calculate the focal lengths of the objective and the eyepiece. (Given that the least distance of distinct vision = 25 cm)
(I) Write Ampere’s circuital law in mathematical form and explain the terms used.
(II) As the current-carrying solenoid is made longer, the magnetic field produced outside it approaches zero. Why?
(III) A flexible loop of irregular shape carrying current, when located in an external magnetic field, changes to a circular shape. Give reason.
The electric field (\( \vec{E} \)) and electric potential (\( V \)) at a point inside a charged hollow metallic sphere are respectively:
Two coherent light waves, each of intensity \( I_0 \), superpose and produce an interference pattern on a screen. Obtain the expression for the resultant intensity at a point where the phase difference between the waves is \( \phi \). Write its maximum and minimum possible values.
The dimensions of ‘self-inductance’ are:
In a single slit diffraction experiment, the aperture of the slit is 3 mm and the separation between the slit and the screen is 1.5 m. A monochromatic light of wavelength 600 nm is normally incident on the slit. Calculate the distance of (I) first order minimum, and (II) second order maximum, from the centre of the screen.
An electric dipole of dipole moment \( \vec{p} \) consists of point charges \( +q \) and \( -q \), separated by distance \( 2a \). Derive an expression for the electric potential in terms of its dipole moment at a point at a distance \( x \, (x \gg a) \) from its centre and lying:
(I) along its axis, and
(II) along its bisector (equatorial) line.
You are provided with a large number of 1 \(\mu\text{F}\) identical capacitors and a power supply of 1200 V. The dielectric medium used in each capacitor can withstand up to 200 V only. Find the minimum number of capacitors and their arrangement required to build a capacitor system of equivalent capacitance of 2 \(\mu\text{F}\) for use with this supply.
With the help of a labelled diagram, explain the principle of working of a moving coil galvanometer. Write the purpose of using (i) radial magnetic field, and (ii) soft iron core, in it.
A parallel plate capacitor with plate area \( A \) and plate separation \( d \) has a capacitance \( C_0 \). A slab of dielectric constant \( K \) having area \( A \) and thickness \( \left(\frac{d}{4}\right) \) is inserted in the capacitor, parallel to the plates. Find the new value of its capacitance.
(i) Write Biot-Savart's law in vector form.
(ii) Two identical circular coils A and B, each of radius \( R \), carrying currents \( I \) and \( \sqrt{3} I \) respectively, are placed concentrically in XY and YZ planes respectively. Find the magnitude and direction of the net magnetic field at their common centre.
(a) Differentiate between 'conduction current' and 'displacement current', giving one similarity and one dissimilarity between them.
(b) Explain the existence of electromagnetic waves in free space, using the concept of displacement current.
Three cells A, B, and C of EMFs 2 V, 3 V, and 5 V respectively are connected in parallel to each other. Their internal resistances are 5 \( \Omega \), 5 \( \Omega \), and 1 \( \Omega \) respectively. Calculate the currents flowing through the cells A, B, and C.
(i) A rectangular loop of sides \( l \) and \( b \) carries a current \( I \) clockwise. Write the magnetic moment \( \vec{m} \) of the loop and show its direction in a diagram.
(ii) The loop is placed in a uniform magnetic field \( \vec{B} \) and is free to rotate about an axis which is perpendicular to \( \vec{B} \). Prove that the loop experiences no net force, but a torque \( \vec{\tau} = \vec{m} \times \vec{B} \).
(a) State Faraday’s law of electromagnetic induction and explain the role of the negative sign in its expression.
(b) Explain, with an example, that Lenz’s law is consistent with the law of conservation of energy.
Write the mathematical forms of three postulates of Bohr’s theory of the hydrogen atom. Using them prove that, for an electron revolving in the \( n \)-th orbit,
(a) the radius of the orbit is proportional to \( n^2 \), and
(b) the total energy of the atom is proportional to \( \frac{1}{n^2} \).
Explain the process of formation of 'depletion layer' and 'potential barrier' in a p-n junction region of a diode, with the help of a suitable diagram. Which feature of junction diode makes it suitable for its use as a rectifier?
(a) Define 'work function' of a metal. How can its value be determined from a graph between stopping potential and frequency of the incident radiation?
(b) The work function of a metal is 2.4 eV. A stopping potential of 0.6 V is required to reduce the photocurrent to zero, in a photoelectric experiment. Calculate the wavelength of light used.