List of top Questions asked in CBSE CLASS XII

When a ray of light propagates from a denser medium to a rarer medium, it bends away from the normal. When the incident angle is increased, the refracted ray deviates more from the normal. For a particular angle of incidence in the denser medium, the refracted ray just grazes the interface of the two surfaces. This angle of incidence is called the critical angle for the pair of media involved.
The figure shows the path of a light ray through a triangular prism. In this phenomenon, the angle \( \theta \) is given by:
Potential difference across a cell in the open circuit is 6 V. It becomes 4 V when a current of 2 A is drawn from it. The internal resistance of the cell is:
A ray of light of wavelength 600 nm is incident in water (\( n = \frac{4}{3} \)) on the water-air interface at an angle less than the critical angle. The wavelength associated with the refracted ray is:
Two cells of emf 2.0 V and 6.0 V and internal resistances 0.1 and 0.4 respectively, are connected in parallel. The equivalent emf of the combination will be:
Using Bohr’s postulates, derive the expression for the radius of the \( n \)-th orbit of an electron in a hydrogen atom. Also find the numerical value of Bohr’s radius \( a_0 \).
Obtain an expression for the torque \( \tau \) acting on a current-carrying loop in a uniform magnetic field \( \vec{B} \). Draw the necessary diagram.
With the help of a circuit diagram, explain the working of a p-n junction diode as a full-wave rectifier. Draw its input and output waveforms.
Five identical cells, each of emf 2 V and internal resistance 0.1 \( \Omega \), are connected in parallel. This combination in turn is connected to an external resistor of 9.98 \( \Omega \). The current flowing through the resistor is:
Dipped in the solution, the electrode exchanges charges with the electrolyte. The positive electrode develops a potential \( V_+ \) (\( V_+>0 \)), and the negative electrode develops a potential \(-V_-\) (\( V_- \geq 0 \)), relative to the electrolyte adjacent to it. When no current is drawn from the cell, then:
When the terminals of a cell are connected to a conductor of resistance \( R \), an electric current flows through the circuit. The electrolyte of the cell also offers some resistance in the path of the current, like the conductor. This resistance offered by the electrolyte is called internal resistance of the cell \( r \). It depends upon the nature of the electrolyte, the area of the electrodes immersed in the electrolyte, and the temperature. Due to internal resistance, a part of the energy supplied by the cell is wasted in the form of heat. When no current is drawn from the cell, the potential difference between the two electrodes is known as emf of the cell \( \varepsilon \). With a current drawn from the cell, the potential difference between the two electrodes is termed as terminal potential difference \( V \).
The carbon isotope \( {}^{12}_6C \) has a nuclear mass of 12.000000 u. Calculate the binding energy of its nucleus.
How does the energy gap of an intrinsic semiconductor effectively change when doped with a (a) trivalent impurity, and (b) pentavalent impurity? Justify your answer in each case.
A convex lens (\( n = 1.52 \)) has a focal length of 15.0 cm in air. Find its focal length when it is immersed in a liquid of refractive index 1.65. What will be the nature of the lens?
A 100-turn coil of radius 1.6 cm and resistance 5.0 \( \Omega \) is co-axial with a solenoid of 250 turns/cm and radius 1.8 cm. The solenoid current drops from 1.5 A to zero in 25 ms. Calculate the current induced in the coil in this duration. (Take \( \pi^2 = 10 \))
Two long, straight, parallel conductors carry steady currents in opposite directions. Explain the nature of the force of interaction between them. Obtain an expression for the magnitude of the force between the two conductors. Hence define one ampere.
(a) On what factors does the speed of an electromagnetic wave in a medium depend?
(b) How is an electromagnetic wave produced?
(c) Sketch a schematic diagram depicting the electric and magnetic fields for an electromagnetic wave propagating along the z-axis.
What is the effect on the interference pattern in Young's double-slit experiment when (i) the source slit is moved closer to the plane of the slits, and (ii) the separation between the two slits is increased? Justify your answers.
Assertion (A): The magnifying power of a compound microscope is negative.
Reason (R): The final image formed is erect with respect to the object.
When a p-n junction diode is subjected to reverse biasing:
Two waves, each of amplitude \( a \) and frequency \( \omega \) emanating from two coherent sources of light superpose at a point. If the phase difference between the two waves is \( \phi \), obtain an expression for the resultant intensity at that point.
Assertion (A): An electron and a proton enter with the same momentum \( \vec{p} \) in a magnetic field \( \vec{B} \) such that \( \vec{p} \perp \vec{B} \). Then both describe a circular path of the same radius.
Reason (R): The radius of the circular path described by the charged particle (charge \( q \), mass \( m \)) moving in the magnetic field \( \vec{B} \) is given by \( r = \frac{mv}{qB} \).
Assertion (A): Lenz's law is a consequence of the law of conservation of energy.
Reason (R): There is no power loss in an ideal inductor.
Assertion (A): Photoelectric current increases with an increase in intensity of incident radiation, for a given frequency of incident radiation and the accelerating potential.
Reason (R): Increase in the intensity of incident radiation results in an increase in the number of photoelectrons emitted per second and hence an increase in the photocurrent.
A resistor and an ideal inductor are connected in series to a \( 100 \sqrt{2} \, \text{V}, 50 \, \text{Hz} \) AC source. When a voltmeter is connected across the resistor or the inductor, it shows the same reading. The reading of the voltmeter is: