Q21) State Huygen's postulates of wave theory. Sketch the wavefront emerging from a  (i) point source of light and (ii) linear source of light like a slit.  (Marks 1+2)

Q22) State the conditions for total internal reflection of light to take place at an interface separating two transparent media. Hence derive the expression for the critical angle in terms of the speeds of light in the two media. (Marks 3)

Q23) State the dependence of work function on the kinetic energy of electrons emitted in a photocell. If the intensity of incident radiation is doubled, what changes occur in the stopping potential and the photoelectric current. (Marks 1+2)

Q24) With the help of a labelled circuit diagram, explain how will you determine the internal resistance of a primary cell using a potentiometer. State the formula used.  (Marks 3)

Q25) A short bar magnet of magnetic moment 0.9 joule/tesla, is placed with its axis at 45^o to a uniform magnetic field. If it experiences a torque of 0.063 joule, (i) calculate the magnitude of the magnetic field and (ii) what orientation of the bar magnet corresponds to the stable equilibrium in the magnetic field ?  (Marks 3)

Q26) A conductor of length 'I' is connected to a d.c source of potential 'V' . If the length of the conductor is tripled, by stretching it, keeping 'V' constant, explain how do the following factors vary in the conductor :
(i) Drift speed of electrons, (ii) Resistance and (iii) Resistivity.  (Marks 3)

Q27) A proton and an alpha particle of the same velocity enter in turn a region of uniform magnetic field acting in a plane perpendicular to the magnetic field. Deduce the ratio of the radii of the circular paths described by the particles. Explain why the kinetic energy of the particle after emerging from the magnetic field remains unaltered.  (Marks 3)

Q28) State the postulates of Bohr's model of hydrogen atom. The electron, in a given Bohr orbit has a total energy of -1.5 eV. Calculate its (i) kinetic energy. (ii) potential energy and (iii) wavelength of light emitted, when the electron makes a transition to the ground state. (Ground state energy = -13.6 eV)  (Marks 5)

Q29) For a given a.c. circuit, distinguish between resistance, reactance and impedance. An a.c. source of frequency 50 hertz is connected to a 50 mH inductor and a bulb. The bulb glows with some brightness . Calculate the capacitance of the capacitor to be connected in series with the circuit , so that the bulb glows with maximum brightness.  (Marks 3+2)

Q30) Drawing a labelled circuit diagram, explain the working principle of a common-emitter transistor amplifier. State the phase relation between input and output signals.
Or
With the help of a labelled circuit diagram, explain how a transistor oscillator works.  (Marks 5)

Useful constants

(I) Planck's constant, h = 6.6 x 10^-34 Js
(ii) Speed of light in air, c= 3 x 10⁸ m s^-1

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