Session Ending Exam, 2021-22
Class-XII
Subject –Physics (042)
Max. Marks 35 Max. Time 2 Hrs
General Instructions:
(i) There are 12 questions in all. All questions are compulsory.
(ii) This question paper has three sections: Section A, Section B and Section C.
(iii) Section A contains three questions of two marks each,
Section B contains eight questions of three marks each,
Section C contains one case study-based question of five marks.
(iv) There is no overall choice. However, an internal choice has been provided in one question of two marks and two questions of three marks. You have to attempt only one of the choices in such questions.
(v) You may use log tables if necessary but use of calculator is not allowed.
SECTION – A
1. Threshold frequency of metal is f0. When light of frequency 2f0 is incident on metal plate, maximum velocity of electron emitted is v1. When frequency of incident radiation is increased to 5f0, maximum velocity of electrons emitted is v2. Find the ratio of v1 to v2.
2. Why is there significant fall in intensity of the secondary maxima compared to the central maximum, whereas in double slit experiment all the bright fringes are of the same intensity.
Ans. The reason is that the intensity of the central maximum is due to the constructive interference of wavelets from all parts of the slit, whereas the first secondary maximum is due to the contribution of wavelets from one-third part of the slit (wavelets from remaining two parts interfere destructively) and the second secondary maximum is due to the contribution of wavelets from the one-fifth part only (the remaining four parts interfere destructively) and so on. Hence, the intensity of secondary maximum decreases with the increases in the order n of the maximum. Whereas in interference, all maxima observed are due to waves from both slit interfere constructively.
OR
(i) State the conditions for total internal reflection
Ans. Condition for total internal reflection
(a) Light rays always passes from denser medium to rarer medium
(b) Angle of incidence should be always greater than critical angle.
(ii) Write the name of two phenomena based on total internal reflection.
Ans. Mirage and optical fibre communication.
3. Calculate the number of photons entering the pupil of our eye per second corresponding to the minimum intensity of white light that we humans can perceive (-10-10 W/m2). Take the area of the pupil to be about 0.4 cm2 and the average frequency of white light to be about 6 x 1014 Hz.
SECTION – B
4. Using the data given below, state which two of the given lenses will you prefer to construct a best possible.
(i) telescope and (ii) compound microscope
Also, indicate which of the selected lenses is to be used as an objected and as an eyepiece in each case.
5. (i) Electromagnetic waves with wavelength
For telescope: Objective should be of large aperture and large focal length, so lens L2 is fit for objective. Eyepiece should be of small aperture and small focal length, so lens L3 is fit for making eyepiece.
(ii) For compound microscope: Objective must be of small aperture and small focal length and therefore lens L3 can be used for objective. Eyepiece must be of small aperture and large focal length as compare to objective, thus lens L1 is fit for making eyepiece.
(a) λ1 is used in satellite communication
Ans. Microscope is used in satellite communications, so λ1 is the wavelength of microwave.
(b) λ2 is used to kill germs in water purifiers.
Ans. Ultraviolet rays are used to kill germs in water purifiers, so λ2 is the wavelength of UV –rays.
(c) λ3 is used to detect leakage of oil in underground pipelines.
Ans. X –rays are used to detect leakage of oil in underground pipelines, so λ3 is the wavelength of X-rays.
(d) λ4 is used to improve visibility in runways, during fog and mist conditions
Ans. Infrared wave is used to improve visibility on runways, during fog and mist conditions, so λ4 is the wavelength of infrared waves.
Identify and name the part of electromagnetic spectrum to which these radiations belong.
(ii) The magnetic field of plane electromagnetic wave is given by
B = 2 x 10-7 sin (0.5 x 103 𝑥 + 1.5 x 1011 t)
Where, all quantities are in their SI units.
Then, write the name of the region to which this electromagnetic wave belongs.
6. Construct a refracted wavefront using Huygens’ principle, when a plane wavefront is incident on a plane surface from an optically denser medium. Further with its help, define critical angle.
Ans.
Ans. When a plane wave front PQ travelling in an optically denser medium of refractive index (μ1) with the speed of (v1) is incident on the surface of rarer medium at an angle of incidence (i), then the wavelets spread in the rarer medium of refractive index (μ2) (where μ2 < μ1) with a speed of (v2) (where μ2 > μ1).
Thus, a refracted wavefront RS is formed as shown in the figure. The refracted wavefront subtends an angle of refraction r. As μ2 > μ1 hence the angle of refraction is greater than the angle of incidence (i.e., r > i). However, Snell’s law holds good, according to which
Critical angle: As angle i increases; value of angle r also increases. If for a certain value of i = ic, the angle of refraction just becomes 900, then
This angle ic is called the critical angle.
OR
The intensity at the central maxima (O) in a Young’s double slit experimental is I0. If the distance OP equals one-third of fringe width of the pattern, then show that the intensity at point would be I0/4
7. i) Define the Q-value of a nuclear process.
Ans. The Q-value of a nuclear process refers to the energy released/absorbed in the given process. It is equal to the difference between the initial mass energy and total mass energy of the products.
ii) An atomic reactor can deliver 100 MW power. The energy released due to fission of each nucleus of 239 Pu is 160 MeV. Calculate the numbers of plutonium atoms fissioned per second.
OR
(i) Write two distinguishing features of nuclear force.
(a) Nuclear forces act between a pair of neutrons, a pair of protons and also between a neutron-proton pair, with the same strength. This shows that, nuclear forces are independent of charge.
(b) The nuclear forces are dependent on spin or angular momentum of nuclei.
(ii) If both the numbers of photons and neutrons are conserved in a nuclear reaction like
In what way, its mass is converted into the energy?
Ans. The sum of masses of nuclei of product element is less than the sum of masses of reactants and hence loss of mass takes place during the reaction. This difference of mass of product elements and reactant gets converted into energy and liberated in the form of heat.
Here, the sum of masses of 10Ne20 and 2He4 is less than the sum of two 6C12 and conversion of this mass defect is used to produce energy
(iii) A neutron is absorbed by 6Li3 nucleus as shown in the reaction below
Calculate the mass defect and energy released (Q) in this reaction.
8. (i) Name the device which is used in cellphone which provides light in its display.
Write and explain the principle on which this device is based.
Ans. Light is produced in cellphone’s display with the help of the device as LED (Light Emitting Diode).
Principle of LED: It is heavily doped forward biased p-n junction diode which converts electrical energy into light energy.
When p-n junction is forward biased, electrons and holes move towards opposite sides of junction through it. Therefore, there are excess of minority carriers on the either side of the junction boundary, which recombines with majority carriers near the junction. On recombination of electrons and holes, the energy is given out in the form of heat and light.
(ii) Why a resistor is used in series with the LED?
Ans. A low voltage DC supply (5V) is required to operate LED, as current drawn by LED’s is of order of milliampere. So, a resistor of suitable value is joined in series with the LED to limit the current upto the save value required
9. With the help of a ray diagram, show the formation of image of a point object by refraction of light at a spherical surface separating two media of refractive indices n1 and n2 (n1 > n2) respectively. Using a diagram, derive the relation
Ans. Let a point object O be placed at a distance u from the pole in a medium of refractive index n1 outside the spherical refracting surface of radius of curvature R. Its real image I is formed at a distance v in denser medium of refractive index n2. Here, angle of incidence is I and the angle of refraction is r.
10. i) The resistivity of pure silicon is 3000 Ω-m and the electron & hole mobilities are 0.12 m2 V-1 s-1 & 0.045 m2 V-1 s-1, respectively. Determine the resistivity of a specimen of the material, when 1019 atoms of phosphorous are added per m3
ii) Assuming that the two diodes D1 and D2 used in the electric circuit as shown in the figure are ideal, find out the value of the current flowing through 1 Ω resistor.
11. i) If the frequency of incident light on a metal surface is doubled, will the kinetic energy of the photoelectrons be doubled? Give reason.
ii) What is the number of photons emitted per second by a medium wave transmitter of 10kW power, emitting radiowaves of wavelength 500 m?
SECTION – C
12. At room temperature, most of the H-atoms are in ground state. When an atom receives some energy (i.e., by electron collisions), the atom may acquire sufficient energy to raise electron to higher energy state. In this condition, the atom is said to be in excited state. From the excited state, the electron can fall back to a state of lower energy, emitting a photon equal to the energy difference of the orbit.
In a mixture of H – He+ gas (He+ is single ionised He-atom), H- atomand He+ ions are excited to their respective first excited states. Subsequently, H – atoms transfer their total excitation energy to He+ – ions (by collisions).
i) In which of the following transition, speed of revolving electron is maximum?
a) P b) Q c) R d) None of these
ii) The wavelength of light emitted in the visible region by He+ -ions after collisions with H – atoms is
(a) 6.5 x 10-7 m (b) 5.6 x 10-7 m (c) 4.8 x 10-7 m (d) 4.0 x 10-7 m
iii) The ratio of kinetic energy of the electrons for the H-atom to that of He+ – ions for n = 2 is
(a) 1/4 (b) 1/2 (c) 1 (d) 1
iv) The radius of the ground state orbit of H- atom is
Ans. (b)
v) Angular momentum of an electron in H-atom in first excited state is
(a) h/π (b) h/2π (c) 2π/h (d) π/h