List of top Questions asked in CBSE CLASS XII

Find the intensity of light at a point on the screen when two interfering waves of the same intensity ($I_0$) have a path difference of (i) $\frac{\lambda}{4}$ and (ii) $\frac{\lambda}{3}$.
Under what conditions the (i) impedance of the circuit is minimum ? (ii) Wattless current flows in the circuit ?
How does the size of a nucleus depend on its mass number A ? Hence prove that the density of nucleus is a constant, independent of A, for all nuclei.
Name the electromagnetic waves which are used (iii) in water purifiers. Also write their wavelength range.
Draw electric field lines and equipotential surfaces for a system of two equal and opposite point charges separated by some distance.
Explain the statement : “Current is a scalar although we represent current with an arrow”.
Consider the following nuclides : $^{12}_6\text{C}$, $^{198}_{80}\text{Hg}$, $^{14}_6\text{C}$, $^{197}_{79}\text{Au}$. Group them into isotopes and isotones.
Derive an expression for the magnetic field $\vec{B}$, due to a circular coil of N turns, each of radius r carrying current I, at a distance ‘x’ from the centre along its axis.
Two long straight parallel conductors A and B carrying steady currents $I_a$ and $I_b$ in the same direction are separated by a distance d. Deduce the expressions for the force acting on length L of conductor B due to conductor A and show it in figure. Write the expression for the force acting on length L of conductor A due to conductor B and show that it follows Newton's third law.
Name the electromagnetic waves which are used (ii) in remote switches for TV sets. Also write their wavelength range.
Find ratio ($\lambda_a/\lambda_p$) of the de Broglie wavelength $\lambda_a$ and $\lambda_p$ associated respectively with an alpha particle and a proton, if they are moving with the same kinetic energy.
Find ratio ($\lambda_a/\lambda_p$) of the de Broglie wavelength $\lambda_a$ and $\lambda_p$ associated respectively with an alpha particle and a proton, just after they are accelerated through the same potential difference.
Define mutual inductance of a pair of coils. Write its SI unit.
A long solenoid of radius R and length L has n turns per unit length. A circular loop of radius r(<R) is placed inside at the centre of the solenoid such that its axis coincides with the axis of the solenoid. Obtain the mutual inductance of the solenoid and the loop.
Name the electromagnetic waves which are used as a diagnostic tool in medicine. Also write their wavelength range.
In a Young’s double-slit experiment, a beam of light consisting of two wavelengths 500 nm and 600 nm is used. The interference fringes are observed at a screen placed 1.8 m away from the plane of slits (slit separation 0.3 mm). Calculate the least distance from the central maximum where the bright fringes due to both the wavelengths coincide.
Assertion (A) : On forward biasing a p-n junction diode, the height of the barrier potential increases.
Reason (R) : In forward biasing of a p-n junction diode, the direction of the applied voltage is in the same direction as the built-in potential.
Calculate the temperature at which the resistance of a conductor becomes 20% more than its resistance at $27^\circ\text{C}$. The value of the temperature coefficient of resistance of the material of conductor is $2.0 \times 10^{-4} ^\circ\text{C}^{-1}$.
A ray of light is incident on the face of a triangular prism of refracting angle $60^\circ$ and it just suffers total internal reflection at the other face. Find the angle of incidence for the ray if the refractive index of the material of the prism is $\sqrt{2}$.
A 5 cm long pencil is placed along the principal axis of a concave mirror of focal length 20 cm such that its nearest end is at a distance of 25 cm from the mirror. Calculate the length of the image of the pencil.
Write two points of difference between intrinsic and extrinsic semiconductors.
Assertion (A) : Two electric heaters of power $P_1$ and $P_2$ ($> P_1$) are joined in series across a dc source of voltage V. The power consumed by the combination will be less than that consumed by $P_1$ when connected across the same source.
Reason (R) : The power consumed by a electric device when connected to a dc source of voltage V is proportional to its resistance.
A galvanometer of resistance $27 \Omega$ is converted into an ammeter of range (0 – 10 mA) using a resistance of $3 \Omega$. The galvanometer will show full scale deflection for a current of about –
The magnetic flux $\phi$ (in Wb) linked with a coil is related to time t (in s) as
$\phi = 5 At^2 + Bt - 2C$
The SI units of A and B are respectively
Assertion (A) : Light added to light can produce darkness.
Reason (R) : When two coherent light waves interfere, there is darkness at position of destructive interference.