PRACTICE QUESTIONS 2021-22
CBSE class 12 Boards
Physics Theory
Term-2
MM.35 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
Q.1 (a) Distinguish between an intrinsic semiconductor and P type semiconductor. Give reason, why a P type semiconductor crystal is electrically neutral, although nh>>ne?
b) State the main practical application of LED. Explain, giving reason, why the semiconductor used for fabrication of visible light LED‘s must have a band gap of at least nearly 1.8 eV.
Q.2 (a) Using Bohr’s second postulate of quantisation of orbital angular momentum show that the circumference of the electron in the n, h orbital state in the hydrogen atom is n times the De-Broglie wavelength associated with it.
b) The wavelength of first line of Lyman series is 1215 Å. Calculate the wavelength of the first line of Balmer series
Or
i) A difference of 2.3eV separates two energy levels in an atom. What is the frequency of radiation emitted when the atom makes a transition from the upper level to the lower level?
ii) Find the relation between the three wavelengths λ1,λ2and λ3 from the energy level diagram shown below-
Q.3 (a) In a pure semiconductor crystal of silicon, if aluminium is added then what type of extrinsic semiconductor is obtained.Draw the energy band diagram of this extrinsic semiconductor so formed.
b) With the help of necessary circuit diagram, explain the working of a photo diode used for detecting optical signals.
Section -B
Q.4 i) Show that the limiting value of the angle of prism is twice it’s critical angle.
ii) Light is incident normally on face AB of a prism as shown in the figure. A liquid of refractive index n is placed on the face AC of the prism. The prism is made of glass of refractive index 3/2. The limit of n for which total internal reflection takes place on the face AC is-
Or
a) Draw a labelled ray diagram to obtain the real image formed by an astronomical telescope in normal adjustment position. Define its magnifying power.
b) You are given three lenses of power 0.5 D, 4D and 10 D to design a telescope. i) Which lens is should be used as objective and eyepiece? Justify your answer ii) Why is the aperture of the objective prefer to be large?
Q.5 a) Sketch the graph showing variation of stopping potential with frequency of incident radiations for two photosensitive material is A and B having threshold frequency is VA > VB. i) In which case is the stopping potential more and why ? ii) Does the slope of the graph depends on the nature of the material used?
b) Estimate the ratio of De-Broglie wavelength associated with deuterons and alpha particle when they are accelerated from rest through the same accelerating potential V.
Q.6 Given are two nuclei -
Justify that these two nuclei have same radii.
b) When 92 U^235 undergoes fission, 0.1% of the original mass is released into energy. How much energy is released by an atom bomb which contains 10kg of 92U^235
Q.7 a) According to classical electromagnetic theory, calculate the initial frequency of the light emitted by the electron revolving around a proton in hydrogen atom. b) Using Rutherford’s model of atom, derive the expression for total energy of the electron in hydrogen atom.
Q.8 Explain the use of PN junction diode as rectifier. Explain working principle of a full wave rectifier using the circuit diagram. Consider a silicon diode with a threshold voltage of -0.7 V, used as a half wave rectifier. Draw the output if the input is a sine wave form of amplitude 2 V and circuit is on in reverse position.
Q.9 a) State and explain Huygens principle. Prove snell’s law of refraction on the basis of Huygens principle. b) In Young’s experiment, the width of the fringes obtained with the light of wavelength 6000 Å is 2.00 mm. What will be the fringe width if the entire apparatus is immersed in a liquid of n= 4/3 ?
Q.10 a) A small bulb is placed at the bottom of the tank containing water to a depth of 80 cm. What is the area of the surface of water through which light from the bulb can emerge out? Refractive index of water is 1.33. Consider the Bulb to be a point source.
b) A point object is placed 60 cm in front of a convex lens of focal length 15 cm. A plane mirror is placed at 10 cm behind a convex lens. Where is the image formed by the system?
Q.11 (a) Electromagnetic waves are believed to carry energy and momentum. Give an illustrative example that proves this characteristic of EM waves.
b) What is the one common function of each of these? Ozone layer, glass windows, special glass goggles used while welding metals.
c) Write an equation that gives the speed of an electromagnetic wave in a material medium in terms of the electric and magnetic properties of the medium. Compare the speed of the em wave in a material medium and in vacuum.
Or
(a) A red light of wavelength λ passes through a single slit of width w and produces a diffraction pattern on the screen. If the red light is replaced by the green light of wavelength 0.75 λ , find the slit width required so as to keep the diffraction pattern for the green light the same as that of the red light.
b) Find the wavelength of the light that produces the first minimum at an angle of 30° through a narrow slit of width 0.8 micrometers.
c) State the formula for the width of the central maximum of the single slit diffraction pattern. What will be the impact on the width of the central maximum if the whole of the diffraction apparatus is immersed in a liquid of refractive index n? Explain.
Section C ( case study)
In total internal reflection 100% of the incident light is reflected back into the denser medium, and there is no loss of intensity, while in the reflection from mirrors and refraction from lenses, there is always some loss of intensity. That is why images formed by total internal reflection are much brighter than those formed by mirrors or lenses. we define total internal reflection as, if the angle of incidence exceeds the critical angle then the total light get internally reflected. Mirage is the best real life example of total internal reflection of light. The highly brilliance of diamonds is only due to the total internal reflection of light through them which is get total internally reflected many times and causes brilliance of diamonds. Prisms are also made by using total internal reflection of light. We all know that, optics is the vast branch of physics which has vast application in our daily life such as optical fibre. Optical fibre are used for long distance transmission of audio and video signal also. And they are mainly constructed on the basis of total internal reflection of light to reduce the decrease in amplitude of the sending signal.
1) The phenomena involved in the reflection of radio waves by ionosphere is similar to-
i) Reflection of the light by a plane mirror
ii) Total internal reflection of the light in air during a mirage
iii) Dispersion of light by water molecules during the formation of a rainbow
iv) Scattering of light by the particles of air
2) How total internal reflection of light would be possible in case of optical fibre?
3) Critical angle of glass is C1 and that of water is C2 critical angle for water and glass surface would be
i) less than C1
ii) between C1 and C2
iii) greater than C2
iv) less than C2
4) If in core, angle of incidence is equal to critical angle, then angle of refraction will be
i) 0°
ii) 45°
iii) 90°
iv) 180°
5) In an optical fibre, correct relation for refractive indices of core and cladding is
i) n1=n2
ii) n1>n2
iii) n1<n2
iv) n1+n2=2
Justify that these two nuclei have same radii.
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