IGNOU BPHCT-137 December 2025 Previous Year Question Paper

Q1.Answer any five parts : 5x2=10

(a) Define numerical aperture.

(b) Draw energy level diagram of four level pumping scheme in lasers.

(c) Diffraction pattern is produced by a grating having 15000 lines per inch. If the wavelength of light used is 500 nm, calculate the order of principal maxima obtained in the diffraction pattern.

(d) Depict intensity distribution in Fresnel diffraction pattern of a wire.

(e) List two experiments to observe diffraction of light outside the physics laboratory.

(f) List any two differences between biprism and Lloyd’s mirror.

(g) State any two properties of a wavefront.

(h) Write down 1-D wave equation for sound waves in a gaseous medium explaining each term.

Q2.Answer any two parts: 2x5=10

(a) Two waves travelling in opposite directions on a string fixed at both ends are described by the equations : J, (x, 0) 5 (0-2 x) sin (2x — 40) and Yo (x, t) = (0-2 n) sin (2x + 40) (@) Obtain the equation of standing wave.

(ii) The string oscillates in only one loop and its one end is fixed at x =

Q3.Calculate the distance between two fixed ends of the string.

(b) State Malus’ law. Unpolarised light is incident on two polarizing sheets placed one over the other. If the intensity of the finally transmitted light is one-fourth of the intensity of the incident light, calculate the angle between the transmission axes of the polarizing sheets.

(c) Explain any five factors which affect acoustics of a building.

Q4.Answer any two parts: 2x5=10

(a) Using sodium light, interference fringes are obtained due to reflection for a thin air wedge. When viewed normally, 10 fringes are seen at a distance of 1 cm. Calculate the angle of the wedge. Take 2 = 5893 x 10-10 m.

(b) Newton’s rings are formed in reflected light with light of wavelength 6000 A when a liquid of refractive index p is placed between the plates. If diameter of 4th bright ring is 0.2 cm and radius of curved surface is 50 cm, calculate p.

(c) Discuss how Michelson interferometer can be used to determine the difference in wavelengths (1 and 2.2 emitted by a source. Assume that 41 and 42 are very close to one another. Derive the expression for the difference in wavelengths for 41 > 3.2.

Q5.Answer any two parts: 2x5=10

(a) A beam of parallel light of wavelength 6000 A is incident normally on a metal sheet having circular aperture of diameter 1.2 mm. The shadow is cast on a variable screen. Calculate the distance of screen at which the aperture will transmit 1 or 3 Fresnel zones.

(b) Show that individual Fresnel zones have same area.

(c) Consider a double slit diffraction arrangement with b = 9 x 10-3 cm, d = 3.5 x 10 cm and A = 6300 A. Calculate the no. of interference minima between diffraction minima on either side of the central maximum. If the screen is placed at a distance of 5m from the diffraction aperture, calculate the fringe width.

Q6.Answer any two parts: 2x5=10

(a) If light of 589 nm wavelength has a wavefrain 20 long, calculate its

(i) coherence length, and

(ii) coherence time.

(b) What do you understand by pulse dispersion ? Calculate pulse dipersion for a step-index fibre after 5 km having core and cladding refractive indices of

Q7.47 and 1.46, respectively.

(c) Explain the difference between step index and gradient index fibres with the help of diagrams. Depict the modes that a step-index multimode and a graded index multimode fibre support. Y, (x, t) 5 (0-2 170 sin (2x — 4)