Question 1

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.

Accepted Answer

To calculate the magnitude of the electric field produced by the sodium lamp, we first need to determine the intensity of the light. Since the lamp emits light uniformly, it can be treated as a point source radiating power spherically. The intensity (I) at a given distance (r) from the source is defined as the power (P) distributed over the surface area of a sphere, given by the formula I = P / (4πr²).

Given the lamp's power P = 40 W. We must assume a distance 'r' from the lamp, as the electri

Question 2

Describe polarisation of light by reflection. How does degree of polarisation vary with angle of incidence of light?

Accepted Answer

Unpolarised light, which contains electric field vibrations in all planes perpendicular to its direction of propagation, undergoes polarisation upon reflection from a dielectric surface. When unpolarised light strikes a transparent medium (like glass or water) at an oblique angle, the reflected light becomes partially plane-polarised. This occurs because the components of the electric field parallel and perpendicular to the plane of incidence are reflected with different efficiencies.

The degre

Question 3

Discuss the concept of missing orders with reference to double slit diffraction pattern.

Accepted Answer

Missing orders refer to the absence of specific interference maxima in a double-slit diffraction pattern. This phenomenon occurs when a principal maximum of the interference pattern coincides exactly with a minimum of the single-slit diffraction pattern.

In a double-slit setup, the observed intensity pattern is a superposition of the interference from two slits and the diffraction from each individual slit. The interference maxima are determined by the path difference between waves from the two

Question 4

Depict spatial evolution of Fresnel diffraction pattern.

Accepted Answer

Fresnel diffraction occurs when the source or observation point is at a finite distance from the diffracting obstacle or aperture, implying that spherical wavefronts illuminate the object. Unlike Fraunhofer diffraction, the parallel ray approximation is not valid here, leading to patterns that are critically dependent on the propagation distance between the aperture and the observation screen.

Close to the diffracting aperture, the observed pattern typically bears a strong resemblance to the ge

Question 5

Obtain expressions for reflection and transmission amplitude coefficients when electric vector associated with a plane monochromatic electromagnetic wave is in the plane of incidence.

Accepted Answer

When a plane monochromatic electromagnetic wave encounters an interface between two dielectric media, a portion of the wave is reflected, and another portion is transmitted. The electric vector of the incident wave can be oriented either perpendicular (s-polarization) or parallel (p-polarization) to the plane of incidence. This question specifically addresses the case where the electric vector is in the plane of incidence, often referred to as p-polarization or transverse magnetic (TM) mode.

To

Question 6

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.

Accepted Answer

Elliptically polarised light results from the superposition of two orthogonal linearly polarised light waves with a constant phase difference between them. Let these two waves propagate along the z-axis, with their electric field components oscillating along the x and y axes, respectively.

Let the x-component of the electric field be E_x = E_1 cos(kz - ωt) and the y-component be E_y = E_2 cos(kz - ωt + φ), where E_1 and E_2 are the amplitudes, k is the wave number, ω is the angular frequency, a

Question 7

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.

Accepted Answer

In a Young's double-slit experiment, interference fringes are formed due to the superposition of coherent light waves. When a thin transparent sheet is introduced in the path of one of the waves, it alters the optical path length of that wave, leading to a shift in the entire interference pattern.

Consider a thin transparent sheet of thickness 't' and refractive index 'μ' placed in the path of the wave originating from slit S₁ (or S₂). In air or vacuum, light travels a geometric path length 't'

Question 8

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 the point of contact is perfect. The diameters of the 10th and 5th dark rings in the reflected light are 4.50 × 10^-3 m and 3.36 × 10^-3 m. Next, the space be

Accepted Answer

The problem describes a Newton's rings experiment, where a plano-convex lens is placed on a flat glass plate, creating an air wedge that produces interference patterns. When monochromatic light is reflected from this setup, concentric dark and bright rings are observed. The diameter of these rings depends on the wavelength of light, the radius of curvature of the lens, and the refractive index of the medium between the lens and the plate. This experiment is a classic method to determine the wave

BPHCT-137 Solved Assignment — January 2026 / July 2026

WAVES AND OPTICS | IGNOU Bachelor of Science (General) (CBCS) (BSCG)

BPHCT-137—January 2026 / July 2026

WAVES AND OPTICS

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BPHCT-137 Assignment Questions — January 2026

Q1. 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.

Q1. b). Describe polarisation of light by reflection. How does degree of polarisation vary with angle of incidence of light?

Q1. c). Discuss the concept of missing orders with reference to double slit diffraction pattern.

Q1. d). Depict spatial evolution of Fresnel diffraction pattern.

Q2. 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.

Q2. 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.

Q3. 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.

Q4. a). A plane light wave of wavelength 580 nm falls on a long narrow slit of width 0.5 mm.

Q4. b). Discuss Rayleigh's criterion for resolving power of an optical instrument. Obtain an expression for resolving power of a microscope.

Q5. 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.

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