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IGNOU BPHCT-137 (January 2026 – December 2026) Assignment Questions
PART A
1. a) The sodium lamp used in a physics laboratory gives out light uniformly. Suppose that the lamp uses 40 W. Calculate the magnitude of electric field.
b) Describe polarisation of light by reflection. How does degree of polarisation vary with angle of incidence of light?
c) Discuss the concept of missing orders with reference to double slit diffraction pattern.
d) Depict spatial evolution of Fresnel diffraction pattern.
2. a) Obtain expressions for reflection and transmission amplitude coefficients when electric vector associated with a plane monochromatic electromagnetic wave is in the plane of incidence.
b) Obtain an expression for elliptically polarised light resulting due to superposition of two orthogonal linearly polarised light waves. Show that plane polarised light and circularly polarised light are special cases of elliptically polarised light.
3. a) Obtain the expression for shift in fringes when a thin transparent sheet is introduced in the path of one of the waves in a double slit interference experiment.
b) A plano-convex lens of radius 1.0 m is placed on an optically flat glass plate and is illuminated by an extended monochromatic source. Assume that the point of contact is perfect. The diameters of the 10th and 5th dark rings in the reflected light are 4.50 × 103 m and 3.36 × 103 m. Next, the space between the lens and the glass plate is filled with a liquid. The diameter of the 5th ring changes to 3.0×103 m. Calculate the refractive index of the liquid when the ring is (i) dark, and (ii) bright, if the wavelength of light is 589 nm.
PART B
4. a) A plane light wave of wavelength 580 nm falls on a long narrow slit of width 0.5 mm. (i) Calculate the angles of diffraction for the first two minima. (ii) How are these angles influenced if the width of slit is changed to 0.2 mm? (iii) If a convex lens of focal length 0.15 m is now placed after the slit, calculate the separation between the second minima on either side of the central maximum.
b) Discuss Rayleigh’s criterion for resolving power of an optical instrument. Obtain an expression for resolving power of a microscope.
5. a) An atomic system consisting of two energy levels, with population of higher energy level less than that of the lower level, is in thermal equilibrium. Show that the absorption of radiation dominates stimulated emission if radiation of appropriate frequency is introduced into the system. Comment on the consequences of this fact for laser action.
b) Two energy levels of an atomic system are separated by energy corresponding to frequency 3.0 × 10 14Hz. Assume that all atoms are in one or the other of these two energy levels, compute the fraction of atoms in the upper energy level at temperature 400 K. Take kB 1.381023JK-1 and h 6.61034 Js.
c) The refractive indices of the core and cladding materials of an optical fibre are 1.51 and 1.39, respectively. Calculate the numerical aperture and light gathering capacity of the fibre.
IGNOU BPHCT-137 (January 2025 – December 2025) Assignment Questions
PART A
1. a) A sinusoidal wave is described by
y (x, t) = 3.0 sin (3.52t − 2.01x) cm
where x is the position along the wave propagation. Determine the amplitude, wave number, wavelength, frequency and velocity of the wave.
b) A stretched string of mass 20 g vibrates with a frequency of 30 Hz in its fundamental mode and the supports are 40 cm apart. The amplitude of vibrations at the antinode is 4 cm. Calculate the velocity of propagation of the wave on the string.
c) Show that superposition of two linearly polarised light waves having different amplitudes and a finite phase difference can be used to produce elliptically plane polarised waves. Also show that the linear polarisation and circular polarisation are special cases of elliptical polarisation.
2. a) What is a biprism? The inclined faces of a glass biprism ( = 1.5) make an angle of 1 with its base. The biprism is illuminated by sodium lamp ( = 589 nm) and the eye piece is at a distance of 1 m from the slit. A convex lens inserted between the biprism and the eye piece gives clear images of coherent sources in the focal plane of the eye piece. If the images are 0.4 cm apart in one case and 0.16 cm apart in the second case, calculate the width of interference fringes observed on the screen.
b) i) Show that the radius of a dark Newton’s ring is directly proportional to the square root of the radius of curvature of the lens used
ii) Newton’s rings are formed in reflected light of wavelength 5890 x 10^-8 cm with a liquid between the plane and curved surfaces. The diameter of the fifth ring is 0.3 cm and the radius of curvature of the curved surface is 100 cm. Calculate the refractive index of the liquid, when the ring is bright.
c) Explain how Michelson interferometer is used to determine the wavelength of light.
PART B
3. a) A vertical single and double slits and illuminated by a point source. Discuss the salient features of their Fraunhofer diffraction patterns. Also, obtain an expression for intensity distribution in case of double slit.
b) ‘Diffraction limits the image forming capability of optical devices’. Discuss the authenticity of this statement for the particular case of a microscope.
4. a) Discuss applications of lasers in medicine and communication.
b) Define numerical aperture and angle of acceptance. An optical fiber has a numerical aperture of 0.20 and cladding refractive index of 1.59. Calculate the refractive index of the core material and the acceptance angle of the fibre in water whose refractive index is 1.33.
c) With the help of a labeled diagram, discuss lasing action of a He-Ne laser.
d) The refractive index of the core and cladding materials of an optical fibre is 1.52 and 1.46, respectively. Calculate the critical angle, numerical aperture and acceptance angle at the air-fibre interface.
IGNOU BPHCT-137 (January 2024 – December 2024) Assignment Questions
