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ISC Class 12 Physics Solved Question Paper 2015

Short Answer Type

41.

Find the minimum wavelength of the emitted X-rays, when an X-ray tube is operated at 50 kV.

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42.

i) Define half-life of a radioactive substance.

ii) Using the equation, N = N_o e^{- $straight lambda$ t}, obtain the relation between half-life (T) and decay constant ( $straight lambda right parenthesis$ of a radioactive substance.

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43.

With the help of a suitable example and an equation, explain the term pair production.

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44.

Draw a labelled diagram of a full wave rectifier. Show how output voltage varies with time, if input voltage is a sinusoidal voltage.

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45.

What is a NAND gate? Write it's truth table?

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Long Answer Type

46.

Derive an expression for intensity of electric field at a point in broadside position or on an equatorial line of an electric dipole.

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47.

Figure below shows a capacitor C, an inductor L and resistor R, connected in series to an ac supply of 220 V.

Calculate:

i) the resonant frequency of the given LCR circuit

ii) current flowing through the circuit

iii) Average power consumed by the circuit.

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48.
On the basis of Huygen's wave theory of light, show that the angle of reflection is equal to the angle of incidence. You must draw a labelled diagram for this deviation.

Laws of Reflection on the basis of Huygen's Wave Theory of Light:

i) Let XY be a reflecting surface at which a wavefront is being incident obliquely.

ii) Let v be the speed of the wavefront and at the time t =0, the wavefront touches the surface XY at A.

iii) After time t, the point B of wavefront reaches the point B’ of the surface.

iv) According to Huygen’s principle each point of wavefront strikes the reflecting surface.

v) It cannot advance further; but the secondary wavelet originating from point A begins to spread in all directions in the first medium with speed v.

vi) As the wavefront AB advances further, its points A 1, A2, A3... etc. strike the reflecting surface successively and send spherical secondary wavelets in first medium.

vii) First of all the secondary wavelet starts from point A and traverses distance AA’ (=vt) in first medium in time t.

viii) The point B of wavefront, after travelling a distance BB’ reaches point B’ (of the surface), from where the secondary wavelet now starts.

ix) Now taking A as centre we draw a spherical arc of radius AA’ (= vt) and draw tangent A’B’ on this arc from point B.

x) As the incident wavefront AB advances, the secondary wavelets starting from points between wavefront A’B’ represents the new position of AB, i.e., A’B’ is the reflected wavefront corresponding to incident wavefront AB.

Now, in right-angled triangles, ABB' and AA'B', we have

angle space ABB apostrophe space equals space angle AA apostrophe straight B apostrophe space equals space 90 to the power of straight o

side BB' = side AA' (bth are equal to vt)

Side AB' is common.

That is, both triangles are congruent.

Angle BAB = angle AB'A'

i.e., incident wavefront AB and reflected wavefront A’B’ make equal angles with the reflecting surface XY.

As the rays are always normal to the wavefront, therefore the incident and the reflected rays make equal angles with the normal drawn on the surface XY.

i.e., angle of incidence i = angle of reflection r

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