Tuesday, 9 January 2018


XII                                                PHYSICS                                                                                                                                                                                                                          MARKS: 70
                                                                                                                                                                                                                                                                                                    DURATION:3 Hrs

The entire question was compulsory and there were 26 questions. The question paper was having been divided in to five sections: Section A, Section B, Section C, Section D, Section E.
Section A – The section had five questions which carried one marks each. 
Section B – The section had five questions which carried two marks each. Where question no 10 were having options either one of them you have to choose.
Section C- The section had 12 questions which carried three marks each and the questions were descriptive and application based. Where question no – 18 were having options either one of them you have to choose.
Section D- Contains one value based question of four marks.
Section E – The section had three questions which carried five marks each. Where the entire question no – 24, 25, 26 were having options either one of them you have to choose
 1.A metal emits photoelectrons when red light falls on it. Will this metal emit photoelectrons when blue light falls on
it? Why?
2. Name the series of hydrogen spectrum which has least wavelength.
3. What is the ratio of velocities of electron in I, II and III Bohr Orbits?
4. At what temperature does a semiconductor behave as an insulator?
5. What is the least size of an antenna required to radiate a signal of wavelength?
6. Define Brewster’s angle. Show that the Brewster’s angle iB for a given pair of media is related to critical angle ic through the relation ic = sin-1(cot iB)
7. Two independent light sources cannot act as coherent sources. Why?
8. Define distance of the closest approach. An α particle of kinetic energy K is bombarded on a thin gold foil. The distance of the closest approach is  r.What will be the distance of closest approach for an α particle of double the kinetic energy?
9. The diagram shows a piece of pure semiconductor S in series with a variable resistor R and a source of constant voltage V. Would you increase or decrease the value of R to keep the reading of ammeter A constant, when semiconductor S is heated ? Give reason.

10. Why is a photo diode used in reverse bias ?
Write the truth table for the combination of gates shown.

11. Complete the ray diagram in the following figure where, n1 is refractive index of medium and n2 is refractive index of material of lens.

12. Explain briefly (i) sparkling of diamond (ii) use of optical fibre in communication.
13. In double slit experiment SS2 is greater than SS1 by 0.25l. Calculate the path difference between two interfering beam from S1 and S2 for minima and maxima on the point P as shown in figure.

14. An object is placed in front of a concave mirror of focal length 20 cm. The image is formed three times the size of the object. Calculate two possible distances of the object from the mirror.
15. What is depletion region in p-n junction diode. Explain its formation with the help of a suitable diagram.
16. What is rectification ? With the help of labelled circuit diagram explain half wave rectification using a junction diode.
17. Explain the following terms :
(1) Ground waves
(2) Space waves
(3) Sky waves
18. A T.V. Tower has height of 70 m.
(i) How much population is covered by the T.V. broadcast if the average population density around the tower is 1000 km-2 ? Radius of earth is 6.4 × 106 m.
(ii) By how much should the height of the tower be increased to double the coverage area ?
A communication system is operating at wavelength l = 750 nm. If only 1% of the frequency is used as channel bandwidth for optical communication then find the number of channels that can be accommodated for transmission of(i) an Audio signal requiring a bandwidth of 8 kHz. (ii) a Video T.V. signal requiring a bandwidth of 4.5 kHz.
19. State the law of radioactive decay. Deduce the relation N = N0e-lt, where symbols have their usual meaning.
20. What is the (i) Speed (ii) Momentum (ii) de-Broglie wavelength of an electron having kinetic energy of 120eV?
21. Why wave theory of light could not explain the photoelectric effect? State two reasons.
22. An electron and a proton are possessing same amount of K.E., which of the two have greater de-Broglie,   wavelength? Justify your answer. 
23. Dev is a good student and studied semi-conductors and electronic devices in school. He also used many semi-conductor devices like LED TV.s, radio etc. He saw his monthly bill of CNG/LPG and discussed with his father that the cost was going up. To cope up with the future he suggested using solar devices. His father also agreed using solar devices.(a) What are the values developed by Dev ? (b) Which materials (any two) would be suitable semi-conductor forusing solar energy ? (c) Explain the working of a solar cell.

24. (i) In Young’s double slit experiment, deduce the conditions for (a), constructive, and (b) destructive interference at a point on the screen. Draw a graph showing variation of intensity in the interference pattern against position x on the screen. (ii) Compare the interference pattern observed in Young’s double slit experiment with single slit diffraction pattern, pointing out three distinguishing features.

(i) Plot a graph to show variation of the angle of deviation as a function of angle of incidence for light passing through a prism. Derive an expression for refractive index of the prism in terms of angle of minimum deviation and angle of prism.
(ii) What is dispersion of light? What is its cause?
(iii) A ray of light incident normally on one face of a right isosceles prism is totally reflected as shown in fig. What must be the minimum value of refractive index of glass? Give relevant calculations.

25. State the condition under which the phenomenon of diffraction of light takes place. Derive an expression for the width of the central maximum due to diffraction of light at a single slit. Also draw the intensity pattern with angular position.
Draw a labelled ray diagram of a compound microscope and explain its working. Derive an expression for its magnifying power.
26. Draw a circuit diagram to study the input and output characteristics of an n-p-n transistor in its common emitter configuration. Draw the typical input and output characteristics and explain how these graphs are used to calculate current amplification factor of the transistor.                         

Two signals A, B as given below are applied as input to (i) AND (ii) NOR and (iii) NAND gates. Draw the output waveform in each case.