CBSE SAMPLE PAPER PHYSICS CLASS 12

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MARKING SCHEME

CLASS XII

PHYSICS THEORY TERM II

SESSION 2021 – 22

MM:35 TIME: 2 Hours

Table of Contents

CBSE SAMPLE PAPER PHYSICS CLASS 12 TERM 2

ANS 1	As given in the statement antimony is added to pure Si crystal, then a n -type extrinsic semiconductor would be so obtained, Since antimony(Sb) is a pentavalent impurity. Energy level diagram of n-type semiconductor Donar Energy level	1 Mark 1 Mark
ANS 2	No	1/2 mark
	Because according to Bohr’s model, En = − 13.6 and electrons having different energies belong to n2 different levels having different values of n.	1/2 mark
	So, their angular momenta will be different, as L = mvr = nh 2஠	1 mark
OR
(i)	The increase in the frequency of incident radiation has no effect on photoelectric current. This is because of incident photon of	1/2

(i) increased energy cannot eject more than one electron from the metal surface.

ANS 3: Photodiodes are used to detect optical signals of different intensities by changing current flowing through them.

1/2 mark

Applications of photodiodes:

In detection of optical signals.
In demodulation of optical signals.
In light operated switches.
In speed reading of computer punched cards.
In electronic counters

(any two out of these or any other relevant application)

(1/2) X 2= 1mark

SECTION B

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ANS 4:

ANS 5: A junction diode allows current to pass only when it is forward biased. So, if an alternating voltage is applied across a diode the current flows only in that part of the cycle when the diode is forward biased. This property is used to rectify alternating voltages and the circuit used for this purpose is called a rectifier.

1 mark

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1 mark

ANS 6:

Number of atoms present in 2 g of deuterium = 6 × 10²³ Number of atoms present in 2.0 Kg of deuterium = 6 × 10²⁶ Energy released in fusion of 2 deuterium atoms

= 3.27 MeV

Energy released in fusion of 2.0 Kg of deuterium atoms

= 3.27 × 6 × 10²⁶ MeV

= 9.81 × 10²⁶ MeV

= 15.696 × 10¹³ J

Energy consumed by bulb per sec = 100 J

Time for which bulb will glow = 15.696×10 /100 s = 4.97 × 10⁴𝑦𝑒𝑎𝑟

1 mark

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ANS 7:

A locus of points, which oscillate in phase is called a wavefront.

A wave front is defined as a surface of constant phase.

Diagram

Proof n1 sin i = n2 sin r (Derivation)

This is the Snell’s law of refraction.

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1 mark

ANS 8(a) :- Diagram of Compound Microscope for the final image formed at D:

(b)

ANS 8(a)

Ray diagram of astronomical telescope when image is formed at infinity.

ANS 11.(a)

Microwaves are suitable for the radar system used in aircraft

(b) navigation.

Range of frequency of microwaves is 108 Hz to 1011 Hz.

If the Earth did not have atmosphere, then there would be absence of greenhouse effect of the atmosphere. Due to this reason, the temperature of the earth would be lower than what it is now.

1 mark

(c) An e.m. wave carries momentum with itself and given by P = Energy of wave(U)/ Speed of the wave(c)

= U/c when it is incident upon a surface it exerts pressure on it.

1 mark

ANS. 11(a)

The total intensity at a point where the phase difference is ∅, is given by 𝐼 = 𝐼₁ + 𝐼₂ + 2√𝐼₁𝐼₂ 𝐶𝑂𝑆 ∅. Here 𝐼₁ and 𝐼₂ are the intensities of two individual sources which are equal.

When ∅ is 0, I = 4𝐼₁.

When ∅ is 90o, I = 0 Thus intensity on the screen varies between 4𝐼₁ and 0.

2 marks

ANS. 11(b) Intensity distribution as function of phase angle, when diffraction of light takes place through coherently illuminated single slit