# Physics NCERT SAMPLE QUESTION PAPER-SEMESTER-II 2021-2022 Class XII

CBSE Class 12 Physics Sample Question Paper Term 2 ## Q1. In a pure semiconductor crystal of Si, if antimony is added then what type of extrinsic semiconductor is obtained. Draw the energy band diagram of this extrinsic semiconductor so formed.

### 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 OR

## Explain how does (i) photoelectric current and (ii) kinetic energy of the photoelectrons emitted in a photocell vary if the frequency of incident radiation is doubled, but keeping the intensity same? Show the graphical variation in the above two cases.

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### (i) The increase in the frequency of incident radiation has no effect on photoelectric current. This is because of incident photon of 1/2 increased energy cannot eject more than one electron from the metal surface. ### (ii) The kinetic energy of the photoelectron becomes more than the double of its original energy. As the work function of the metal is fixed, so incident photon of higher frequency and hence higher energy will impart more energy to the photoelectrons. ## Q4. Derive an expression for the frequency of radiation emitted when a hydrogen atom de-excites from level n to level (n – 1). Also show that for large values of n, this frequency equals to classical frequency of revolution of an electron.

### ANS 4 From Bohr’s theory, the frequency f of the radiation emitted when an electron de – excites from level nଶ to level nଵis given as ## Q5. Explain with a proper diagram how an ac signal can be converted into dc ( pulsating)signal with output frequency as double than the input frequency using pn junction diode. Give its input and output waveforms.

### 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.  ## Q6. How long can an electric lamp of 100 W be kept glowing by fusion of 2 kg of deuterium? Take the fusion reaction as ### ANS 6 Number of atoms present in 2 g of deuterium = 6 × 10ଶଷ Number of atoms present in 2.0 Kg of deuterium = 6 × 10ଶ଺ Energy released in fusion of 2 deuterium atoms = 3.27 MeV Energy released in fusion of 2.0 Kg of deuterium atoms OR

OR

## (a) Draw a ray diagram of Astronomical Telescope for the final image formed at infinity. (b) A small telescope has an objective lens of focal length 140 cm and an eyepiece of focal length 5.0 cm. Find the magnifying power of the telescope for viewing distant objects when (i) the telescope is in normal adjustment, (ii) the final image is formed at the least distance of distinct vision.

### Diagram of Compound Microscope for the final image formed at D: ### (b) m୭ = 30, f୭ = 1.25 cm, fୣ = 5 cm when image is formed at least distance of distinct vision, D = 25cm Angular magnification of eyepiece OR

### (b) (i) In normal adjustment : Magnifying power. m = f୭/fୣ = (140/5) = 28 (ii) When the final image is formed at the least distance of distinct vision (25 cm) : OR

## (a) “If the slits in Young’s double slit experiment are identical, then intensity at any point on the screen may vary between zero and four times to the intensity due to single slit”. Justify the above statement through a relevant mathematical expression. (b) Draw the intensity distribution as function of phase angle when diffraction of light takes place through coherently illuminated single slit.

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### ANS. 11 (b) Intensity distribution as function of phase angle, when diffraction of light takes place through coherently illuminated single slit ## Q12. CASE STUDY: MIRAGE IN DESERTS    ### (c) Ans (iv) n1 > n2 The refractive index of the core should be greater than the refractive index of the cladding. 