Science (English Medium) Class 12 - CBSE Question Bank Solutions for Physics

(i) Consider a thin lens placed between a source (S) and an observer (O) (Figure). Let the thickness of the lens vary as `w(b) = w_0 - b^2/α`, where b is the verticle distance from the pole. `w_0` is a constant. Using Fermat’s principle i.e. the time of transit for a ray between the source and observer is an extremum, find the condition that all paraxial rays starting from the source will converge at a point O on the axis. Find the focal length.

(ii) A gravitational lens may be assumed to have a varying width of the form

`w(b) = k_1ln(k_2/b) b_("min") < b < b_("max")`

= `k_1ln (K_2/b_("min")) b < b_("min")`

Show that an observer will see an image of a point object as a ring about the center of the lens with an angular radius

`β = sqrt((n - 1)k_1 u/v)/(u + v)`

Figure shows a standard two slit arrangement with slits S₁, S₂, P₁, P₂ are the two minima points on either side of P (Figure). At P₂ on the screen, there is a hole and behind P₂ is a second 2-slit arrangement with slits S₃, S₄ and a second screen behind them.

For light diverging from a point source ______.

1. the wavefront is spherical.
2. the intensity decreases in proportion to the distance squared.
3. the wavefront is parabolic.
4. the intensity at the wavefront does not depend on the distance.

Is Huygen’s principle valid for longitudinal sound waves?

Consider a point at the focal point of a convergent lens. Another convergent lens of short focal length is placed on the other side. What is the nature of the wavefronts emerging from the final image?

What is the shape of the wavefront on earth for sunlight?

Consider Figure for photoemission.

How would you reconcile with momentum conservation? Note light (photons) have momentum in a different direction than the emitted electrons.

O₂ molecule consists of two oxygen atoms. In the molecule, nuclear force between the nuclei of the two atoms ______.

Would the Bohr formula for the H-atom remain unchanged if proton had a charge (+4/3)e and electron a charge (−3/4)e, where e = 1.6 × 10^–19C. Give reasons for your answer.

Assume that there is no repulsive force between the electrons in an atom but the force between positive and negative charges is given by Coulomb’s law as usual. Under such circumstances, calculate the ground state energy of a He-atom.

The Bohr model for the H-atom relies on the Coulomb’s law of electrostatics. Coulomb’s law has not directly been verified for very short distances of the order of angstroms. Supposing Coulomb’s law between two opposite charge + q₁, –q₂ is modified to |F| = `(q_1q_2)/((4πε_0)) 1/r^2, r ≥ R_0 = (q_1q_2)/(4πε_0) 1/R_0^2 (R_0/r)^ε, r ≤ R_0` Calculate in such a case, the ground state energy of a H-atom, if ε = 0.1, R₀ = 1Å.

Explain why elemental semiconductor cannot be used to make visible LEDs.

The frequency response curve (Figure) for the filter circuit used for production of AM wave should be ______.

(i)	(ii)
(iii)	(iv)

1. (i) followed by (ii).
2. (ii) followed by (i).
3. (iii).
4. (iv).

ASSERTION (A): The electrical conductivity of a semiconductor increases on doping.

REASON (R): Doping always increases the number of electrons in the semiconductor.

What is the nuclear radius of ¹²⁵Fe, if that of ²⁷Al is 3.6 fermi?

Draw a graph showing the variation of the number of particles scattered (N) with the scattering angle θ in the Geiger-Marsden experiment. Why only a small fraction of the particles are scattered at θ > 90°?

Give an example each of a metal from which photoelectric emission takes place when irradiated by

1. UV light
2. visible light.

Name the factors on which photoelectric emission from a surface depends.

A narrow beam of protons, each having 4.1 MeV energy is approaching a sheet of lead (Z = 82). Calculate:

1. the speed of a proton in the beam, and
2. the distance of its closest approach

The work function of a metal is 2.31 eV. Photoelectric emission occurs when the light of frequency 6.4 × 10¹⁴ Hz is incident on the metal surface. Calculate

1. the energy of the incident radiation,
2. the maximum kinetic energy of the emitted electron and
3. the stopping potential of the surface.