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THE INDIAN COMMUNITY SCHOOL, KUWAIT

TIME ALLOWED : 3 HOURS NAME OF STUDENT : …………………………………………………

MAX. MARKS : 70 ROLL NO. : ……………….. CLASS/SEC : ……………..

NO. OF PAGES : 5

PHYSICS

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

General Instructions:

(i) All questions are compulsory. There are 27 questions in all.

(ii) This question paper has four sections : Section A, Section B, Section C and Section D.

(iii)Section A contains five questions of one mark each, Section B contains seven questions of

two marks each, Section C contains twelve questions of three marks each, and Section D

contains three questions of five marks each.

(iv) There is no overall choice. However, an internal choice has been provided in one question

of two marks, one question of three marks and all the three questions of five marks

weightage. You have to attempt only one of the choices in such questions.

(v) You may use the following values of physical constants wherever necessary:

c = 3 x 108 ms-1

e = 1.6 x10-19C

µo = 4π x 10-7 TmA-1

εo = 8.85 x 10-12C2N-1m2

1

4𝜋𝜀𝑜 = 9 x 109 Nm2C-2

Mass of electron (me) = 9.1x10-31 kg

Mass of proton (mp) = 1.67 x 10-27 kg

Section A

1. Draw graphs showing variation of photoelectric current with applied voltage for two incident

radiations of equal frequency and different intensities. Mark the graph for the radiation of higher

intensity.

1

CODE : N 042 SERIES : II TERM /FN/ 2018 - 2019

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2. Two signals A and B are used as inputs of a NOR gate. Draw the output wave form.

1

3. State Brewster’s law.

1

4. The line AB in the ray diagram represents a lens. State whether the lens is convex or concave.

1

5. Name the essential components of a communication system.

1

Section B

6. A ray of light incident on an equilateral glass prism propagates parallel to the base line of the

prism inside it. Find the angle of incidence of this ray. Given refractive index of material of glass

prism is √3.

2

7. Write two characteristic features in photoelectric effect which cannot be explained on the basis of

wave theory of light but can be explained only using Einstein’s equation.

2

8. A proton and an α-particle have the same de-Broglie wavelength. Determine the ratio of (i) their

accelerating potentials (ii) their speeds.

2

9. Explain with reason, how the power of a diverging lens changes when (i) it is kept in a medium

of refractive index greater than that of the lens. (ii) incident red light is replaced by violet light.

2

10. Draw the intensity pattern for single slit diffraction and double slit interference. Hence, state two

differences between interference and diffraction patterns.

2

OR

Unpolarised light of intensity I0 is passed through a polaroid P1. When this polarised beam passes

through another polaroid P2 and if the pass axis of P2 makes angle θ with the pass axis of P1, then

write the expression for the polarised beam passing through P2. Draw a plot showing the variation

of intensity when θ varies from 0 to 2π.

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11. In the block diagram of a receiver, identify the boxes labelled as X and Y and write their functions.

2

12. A 12·5 eV electron beam is used to excite a gaseous hydrogen atom at room temperature.

Determine the wavelengths and the corresponding series of the lines emitted.

2

Section C

13. Draw diagram to show the behavior of plane wavefronts as they (a) pass through a thin prism,

(b) pass through a thin convex lens (c) reflected by a concave mirror.

3

14. a) A student wants to use two p-n junction diodes to convert alternating current into direct

current. Draw the labelled circuit diagram she would use and explain how it works.

b) Give the truth table and circuit symbol for NAND gate.

3

15. Name the important process that occurs during the formation of a p-n junction. Explain briefly,

with the help of a suitable diagram, how a p-n junction is formed. Define the term barrier potential.

3

16. If light of wavelength 412·5 nm is incident on each of the metals given below, which ones will

show photoelectric emission and why?

3

17. In Young’s double slit experiment, the two slits are separated by a distance of 1·5 mm and the

screen is placed 1 m away from the plane of the slits. A beam of light consisting of two

wavelengths 650 nm and 520 nm is used to obtain interference fringes. Find

a) the distance of the third bright fringe for λ = 520 nm on the screen from the central

maximum.

b) the least distance from the central maximum where the bright fringes due to both the

wavelengths coincide.

3

18. Draw a block diagram of a simple modulator for obtaining amplitude modulated signal.

A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak

voltage of the modulating signal in order to have a modulation index of 75%?

3

19. Draw a labelled ray diagram to show the image formation in a refracting type astronomical

telescope in the normal adjustment position. Write two drawbacks of refracting type telescopes.

3

OR

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a) Define resolving power of a telescope. Write the factors on which it depends.

b) A telescope resolves whereas a microscope magnifies. Justify the statement.

20. The figure shows the plot of binding energy (BE) per nucleon as a function of mass number A.

The letters A, B, C, D and E represent the positions of typical nuclei on the curve. Point out, giving

reasons, the two processes (in terms of A, B, C, D and E), one of which can occur due to nuclear

fission and the other due to nuclear fusion.

Identify the nature of the radioactive radiations emitted in each step of the decay process given

below.

3

21. Trace the rays of light showing the formation of an image due to a point object placed on the axis

of a spherical surface separating the two media of refractive indices n1 and n2. Establish the relation

between the distances of the object, the image and the radius of curvature from the central point

of the spherical surface.

3

22. Briefly explain the three factors which justify the need of modulating low frequency signal into

high frequencies.

3

23. Three light rays red (R), green (G) and blue (B) are incident on a right-angled prism ‘abc’ at face

‘ab’. The refractive indices of the material of the prism for red, green and blue wavelengths are

1.39, 1.44 and 1.47 respectively. Out of the three which colour ray will emerge out of face ‘ac’?

Justify your answer. Trace the path of these rays after passing through face ‘ab’.

3

24. a) Using Bohr’s second postulate of quantization of orbital angular momentum show that the

circumference of the electron in the nth orbital state in hydrogen atom is n times the de Broglie

wavelength associated with it.

b) Define ionization energy. What is its value for a hydrogen atom?

3

Section D

25. a) Derive the expression for the law of radioactive decay of a given sample having No nuclei

decaying to the number N present at any subsequent time t.

b) Plot a graph showing the variation of the number of nuclei versus the time t lapsed.

c) Two nuclei have mass numbers in the ratio 1:2. What is the ratio of their nuclei densities.

5

OR

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a) Define the terms (i) half life and (ii) average life.

b) Derive expression for average life of a radioactive nuclei. Give its relationship with halflife.

c) A radioactive isotope has a half-life of 5 years. After how much time is its activity reduced to

3.125% of its original value.

26. a) What is a wavefront?

b) Distinguish between a plane wavefront and spherical wavefront.

c) A plane wavefront approaches a plane surface separating two media. If medium 'one’ is

optically denser and medium 'two’ is optically rarer, using Huygens’ principle, explain and

show how a refracted wavefront is constructed. Hence verify Snell's law.

d) When a light wave travels from rarer to denser medium, the speed decreases. Does it imply

reduction its energy? Explain.

5

OR

In Young’s double slit experiment, deduce the condition for constructive and destructive

interference pattern at a point on the screen. Hence obtain the expression for the fringe width.

What is the effect on the interference pattern in Young’s double slit experiment when:

(i) screen is moved close to the plane of the slits?

(ii) separation between the two slits is increased?

Explain your answers in each case.

27. a) Draw the circuit diagram to determine the characteristics of an npn transistor in common

emitter configuration.

b) Explain, using V-I characteristics, how the collector current changes with the base current.

How can (i) output resistance and (ii) current amplification factor be determined from the V-I

characteristics?

5

OR

a) Why are photodiodes preferably operated under reverse bias when the current in the forward

bias is known to be more than that in reverse bias?

b) The two optoelectronic devices: - Photodiode and solar cell, have the same working principle

but differ in terms of their process of operation. Explain the difference between the two devices

in terms of (i) biasing, (ii) junction area and (iii) I-V characteristics.

Class:12

INDIAN COMMUNITY SCHOOL, KUWAIT

SECOND TERM 2018-2019

PHYSICS (042)

Time:3 hours Marks: 70

Marking Scheme

Q

No. Expected Answer/Value Points Marks Total

Marks

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28.

The graph 𝐼2 corresponds to radiation of higher intensity

[Note: Deduct this ½ mark if the student does not show the two graphs

starting from the same point.]

(Also accept if the student just puts some indicative marks, or words, (like cross,

higher intensity) on the graph itself.

½

½

1

29.

[Note: If students write truth table correctly then award ½ mark.]

1

30. When unpolarised light is incident on the surface separating two media, the reflected

light gets (completely) polarized only when the reflected light and refracted light

become perpendicular to each other.

Alternatively

tan of polarizing angle is equal to refractive index. tan ip = n

1 1

31. Concave Lens

Alternatively

It can be convex when the surrounding medium is of higher refractive index.

(Award one mark if the student writes the lens as a convex lens and gives the

reason for this)

1

32. Transmitter, medium or channel and receiver. 1

33.

½

½

½

½

2

½

1

½

34. i. Kinetic energy of emitted electrons are found to be independent of intensity of

incident light.

1 + 1 2

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ii. Below a certain frequency (threshold) there is no photo-emission.

iii. Spontaneous emission of photo-electrons. (Any two)

35.

½

½

2

½

½

36.

½

½

½

½

2

37.

½

2

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½

½ +

½

OR

1

1

2

38. Here X is IF stage. Its function is to change the carrier frequency to a low frequency.

1 2

Y – Amplifier. Its function is to amplify the signal because the detected signal may

not be strong enough to use by the user.

1

39.

½

½

½

2

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Lyman series and Balmer series.

½

40. Diagram – Wave front through a prism 1 3

Wavefront through a convex lens 1

Wavefront reflected by a concave mirror.

1

41.

a)

i) During one half cycle (of the input ac) diode 𝐷1

alone gets forward biased and conducts. During the other half cycle, it is

diode 𝐷2 (alone) that conducts.

ii) Because of the use of the center tapped transformer the current though the load

flows in the same direction in both the half

cycles.

Hence we get a unidirectional/ direct current through the load, when the input is

alternating current.

[Alternatively: The student may just use the following diagrams to

1

1

3

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explain the working.]

b) The circuit symbol, and the truth table, for the NAND gate, are

given below.

1

42. Drift and diffusion

½ +

½

3

Explanation – formation of p-n junction

1

Definition – potential barrier.

1

43. The energy of a photon of incident radiation is given by

Hence, only Na and K will show photoelectric emission

[Note: Award this ½ mark even if the student writes the name of

only one of these metals]

Reason: The energy of the incident photon is more than the work function of only

these two metals.

½

1

½

½

½

3

44.

½

½

3

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½

½

1

45.

2

½

½

3

46.

2 3

Drawbacks:

(i)Large sized lenses are heavy and difficult to support

(ii) large sized lenses suffer from chromatic and spherical aberration.

½

½

OR

a) Resolving power of a telescope is the reciprocal of the smallest angular separation

between the two objects which can be just distinctly seen.

Factors: diameter of the objective,

wavelength of the incident light

2

½

½

3

(b) a telescope produces image of far objects nearer to our eye. Objects which are

not resolved at far distance, can be resolved by telescope. A microscope, on the other

hand magnifies objects nearer to us and produces their large image.

1

47.

a) Nuclear fission of E to D and C; as there is an increase in binding energy per

nucleon

b) Nuclear fusion of A and B into C; as there is an increase in binding energy per

nucleon

b) First step - ∝ particle

Second step – 𝛽 particle

1

1

½

½

3

48. Diagram ½ 3

Derivation- refraction through a spherical surface 2 ½

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49.

Size of antenna – The antenna should have a size comparable to the wavelength of

signal (at least 𝜆/4). For low frequency (unmodulated) signal 𝜆 may be a few km. It

is not possible to have such a long antenna. Hence low frequency transmission

is not possible directly.

1 3

Power radiated by antenna – Power radiated by an antenna of length l is proportional

to (l/𝜆)2. Therefore, for same l, power radiated increases with decreasing 𝜆 i.e.

increasing frequency. Hence, for low frequency signal, power radiated by antenna is

very small and good transmission of signal is not possible.

1

Mixing up of signals: All the low frequency (baseband) signals from various

transmitters, can get mixed up because they have the same frequency range. They can

be separated only if communication is done at high frequency and different band of

frequencies are allotted to different transmitters.

1

50. Angle of incidence at face ac for all three colours, i = 45° Refractive index

corresponding to critical angle 45° is

The ray will be transmitted through face ‘ ac’ if i < ic . This condition is satisfied for

red colour (m = 1.39). So only red ray will be transmitted, Blue and Green rays will

be totally reflected.

½

1

½

1

3

51. a) Proof – Bohr’s second postulate from de Broglie’s hypothesis. 2 3

b) Definition – ionization energy. ½

-13.6 eV ½

52. Law of radioactive decay Derivation 3 5

Graph 1

1:1, Density is independent of mass. 1

OR

Definition, half life and mean life ½ +

½

5

Derivation average life of a radioactive sample. 2

Relation between average life and half life ½

R/Ro = (

1

2)

𝑛

½

3.125/100 = (

1

2)

𝑛

n = 5 ½

N = t/T ½

T ½ = 5x5 = 25 years.

½

53. Wavefront definition 1 5

Difference between plane wavefront and spherical wavefront ½ +

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½

Diagram ½

Derivation: snell’s law. 1 ½

No. Because energy of wave depends on its frequency and not on its speed 1

OR

Diagram Young’s double slit experiment ½ 5

Condition for constructive and destructive interference pattern - Derivation 2

Fringe width ½

β is directly proportional to D. Therefore when D decreases, fringe width decreases. 1

β is inversely proportional to d. Therefore when d is increased, fringe width

decreases.

1

54. Labelled circuit diagram 2 5

Output characteristics curve 1

Explanation, output characteristics 1

Output resistance is the ratio of change in collector-emitter voltage to the change in

collector current.

½

Current amplification factor is ratio of change in collector current to the change in

base current at constant collector- emitter voltage.

½

OR

a) The fractional change in majority charge carriers is very less compared to the

fractional change in minority charge carriers on illumination.

1 5

Photodiode Solar cell

Biasing Used in reverse bias No external bias is given

Junction area Small Large for solar radiation

to be incident on it.

V - I characteristics

1

1

2