Science (English Medium) इयत्ता १२ - CBSE Question Bank Solutions

(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)`

Are the nucleons fundamental particles, or do they consist of still smaller parts? One way to find out is to probe a nucleon just as Rutherford probed an atom. What should be the kinetic energy of an electron for it to be able to probe a nucleon? Assume the diameter of a nucleon to be approximately 10^–15 m.

A nuclide 1 is said to be the mirror isobar of nuclide 2 if Z₁ = N₂ and Z₂ = N₁. (a) What nuclide is a mirror isobar of 11²³ Na? (b) Which nuclide out of the two mirror isobars have greater binding energy and why?

Deuteron is a bound state of a neutron and a proton with a binding energy B = 2.2 MeV. A γ-ray of energy E is aimed at a deuteron nucleus to try to break it into a (neutron + proton) such that the n and p move in the direction of the incident γ-ray. If E = B, show that this cannot happen. Hence calculate how much bigger than B must E be for such a process to happen.

Before the neutrino hypothesis, the beta decay process was throught to be the transition, `n -> p + vece`. If this was true, show that if the neutron was at rest, the proton and electron would emerge with fixed energies and calculate them. Experimentally, the electron energy was found to have a large range.

A rectangular, a square, a circular and an elliptical loop, all in the (x - y) plane, are moving out of a uniform magnetic field with a constant velocity `vecv = vhati`. The magnetic field is directed along the negative z-axis direction. The induced emf, during the passage of these loops, out of the field region, will not remain constant for ______.

Distinguish between isotopes and isobars.

Two nuclei have different mass numbers A₁ and A₂. Are these nuclei necessarily the isotopes of the same element? Explain.

James Chadwick, in 1932 studied the emission of neutral radiations when Beryllium nuclei were bombarded with alpha particles. He concluded that emitted radiations were neutrons and not photons. Explain.

Two nuclei may have the same radius, even though they contain different numbers of protons and neutrons. Explain.

A concave mirror of focal length 12 cm forms three times the magnified virtual image of an object. Find the distance of the object from the mirror.

A converging lens has a focal length of 10 cm in air. It is made of a material with a refractive index of 1.6. If it is immersed in a liquid of refractive index 1.3, find its new focal length.

A point charge +Q is placed at point O, as shown in the figure. Is the potential difference V_A – V_B positive, negative or zero?

Why are microwaves considered suitable for radar systems used in aircraft navigation?

How are the magnetic field lines different from the electrostatic field lines?

How are infrared waves produced?

What role dose infra-red radiation play in maintain the Earth’s warmth

What role dose infra-red radiation play in physical therapy?

Identify the part of the electromagnetic spectrum which is suitable for radar system used in aircraft navigation.

Identify the part of the electromagnetic spectrum which is:

produced by bombarding a metal target by high speed electrons.