IGNOU BPHCT-137 December 2022 Previous Year Question Paper

Q1.Attempt any five parts : 5x2=10

(a) What is the distance between two consecutive points in same phase on a waveform graph of a wave called ? Show it in a labelled diagram.

(b) Is it true that, in the standing wave formed on a string, the displacement and velocity of the particles of the medium (string) are zero at the nodes ? Justify your answer.

(c) Plot the intensity distribution for the interference fringe pattern produced by two waves of the same frequency.

(d) Using sodium light of wavelength 5893 A, interference fringes are formed due to reflection from a thin air wedge. When viewed perpendicularly, 20 fringes are observed in a distance of 2:0 cm. Calculate the angle of the wedge. (Take 1 = 1 for air)

(e) What is the phase difference between secondary wavelets emanating from two consecutive half-period zones ?

(f) Draw the intensity distribution curve for the diffraction pattern due to a straight edge.

(g) Draw the energy level diagram for a four-level pumping scheme for lasers.

(h) State the difference between Step Index Single Mode (SISM) and Step Index Multimode (SIMM) optical fibres.

Q2.Attempt any fwo parts: 2x5=10

(a) Starting from the expression for the steady-state amplitude of a damped forced oscillator show that the resonance frequency is equal to the natural frequency of the driven oscillator. 5

(b) On the basis of Huygens’ principle, explain the concept of wavefront. Also, explain the propagation of light (electromagnetic wave) using the concept of wavefront. 243 BPHCT-137 2

(c) A beam of unpolarised light is incident on a polariser-analyser set-up. When the transmission axes of the polariser and analyser are parallel, the intensity of light emerging from the analyser is 0:4 Wm. What will the intensity of the emergent light be when the angle between the transmission axes of the polariser and analyser is 45° ?

Q3.Attempt any fwo parts : 2x5=10

(a) Two waves of same frequency and constant phase difference have intensities in the ratio 121:1. When superposed, these waves produce interference fringes. Calculate the ratio of the maximum to minimum intensity. 5

(b) A ray of light is incident on a thin film of thickness t and refractive index ut (> 1). The angle of refraction for the ray is r. Obtain the condition for destructive interference. 5

(c) Explain how circular fringes are formed in Michelson interferometer.

Q4.Attempt any éwo parts: 2x5=10

(a) The radius of the first half-period zone of a zone plate is 0-4 mm. When a light of wavelength 6x 10->cm is incident on the zone plate, it brings the rays of light to focus at its brightest point. Calculate the focal length of the zone plate. 5

(b) In a double-slit diffraction set-up, the wavelength of light used is 6 x 10 cm, slit width is 0:02 mm and the spacing between the slits is 0-12 mm. If the distance of the screen from the slits is 1:25 m, calculate the distance between the two consecutive dark fringes. 5

(c) The expression for intensity for the diffraction pattern of a grating having N slits is

Q5.=-A2 sin? B . sin? Ny ९ p? sin? y On the basis of the above relation, obtain the condition for principal maxima in the diffraction pattern.

Q6.Attempt any two parts: 2x5=10

(a) Laser radiation of 5000 A wavelength is sustained in a cavity in which mirrors are separated by a distance of 25 cm.

(i) Estimate the number of modes (n) sustained.

(ii) Calculate the mode separation. 342

(b) What is pulse (or modal) dispersion in optical fibre ? How can it be reduced by graded index fibre ? 243

(c) Explain the operation of Ruby laser with the help of a schematic diagram. 5 BPHCT-137 4 a A (thin air wedge) 4 Wadd #4 पर al milo? 02)? 4 4b202

Q7.=-A2 sin? B sin? Ny