IGNOU BPHCT-137 December 2024 Previous Year Question Paper

Q1.Answer any two parts : 2x5=10

(a) Write Maxwell’s equations for free space and use these to show that wave equation for electric field is : > 27 Vv E |1080 न

(b) State Huygens’ principle and write down the properties of wavefronts.

(c) Two waves of same frequency and constant phase difference have intensities in the ratio 81: 1. On superposition, these waves produce interference fringes. Calculate the ratio of the maximum and minimum intensities.

Q2.Answer any two parts: 2x5=10

(a) Ifthe light of 6000 A has a wave train 20 2 long, calculate its coherence length and coherence time. 5

(b) What is a step-index fibre ? Obtain an expression for the angle of incidence beyond which light will get refracted into the cladding material. What will happen if the refractive index of the cladding is higher than that of the core ? 14+8+1

(c) Describe how Michelson interferometer is used to determine the difference in the wavelengths of the doublet emitted by sodium.

Q3.Answer any one part : 1x10=10

(a)

(i) Discuss spatial evolution of Fresnel diffraction pattern to Fraunhoffer diffraction pattern. 3

(i) Show that a zone plate acts like a multi-foci converging lens. 5

(ii) Depict plot of intensity distribution of Fresnel diffraction pattern of a straight edge. 2

(b)

(i) Newton’s rings are formed in reflected light of wavelength 5800 x 10-8 cm with a liquid between the plane and curved surfaces. The diameter of the fifth ring is 0.8 cm and the radius of curvature of the curved surface is 100 cm. Calculate the refractive index of the liquid if the ring is bright. 5 Gi) A film of refractive index 1.55 and thickness 0.25 mm is introduced in the path of one of the interfering beams in a Michelson interferometer. As a result, 500 dark fringes sweep across the field. Calculate the wavelength of light used.

Q4.Answer any two parts: 2x5=10

(a) State the salient features of observed pattern and intensity distribution pattern of a Fraunhoffer diffraction of a single vertical slit from a point source. 5

(b) Name the three components of a laser required for its operation. Discuss the feedback mechanism used for obtaining coherent laser beam. 1+4

(c) (४0 Depict propagation modes and refractive index profiles of step-index optical fibres. 3 Gi) What do you understand by the term temporal coherence ?

Q5.Answer any two parts : 2x5=10

(a) What is birefringence ? Discuss how a calcite crystal is used to polarise light. 1+4

(b) The wavelength of the standing wave on a guitar string vibrating in its fundamental mode is 2 m. If the speed of wave on the string is 400 ms”, what is the frequency of the second overtone ? 5

(c) A straight edge is illuminated by a narrow slit source kept at a distance of 15 cm. Calculate the distance between the first and second dark bands of the diffraction pattern on a screen kept at a distance of

Q6.0 m from the edge. Take the value of wavelengths, 1 =6x10°cm. 5 27 V.E= [1080 =