Principles of Engineering Physics 1

Introduction

In a homogeneous medium, light travels in a straight line – if an object is placed in its path, it should cast a sharp shadow of the object. However, the shadow of a fine wire caused by sunlight is not observable on a screen and the shadow of a straight edge as shown in Fig. 3.1 is not sharp when observed minutely. Some light bends into the shadow of the straight edge with the intensity of the light decreasing rapidly as we move into the shadow. The amount of bending depends upon the wavelength of the light and the size of the obstacle. In the region of the fuzzy boundary of the shadow of the straight edge, alternate dark and bright fringes of unequal spacings are seen when viewed by a well-focused microscope against a good contrast background. This optical phenomenon of bending of light around the obstacle is called diffraction and the fringes are called diffraction bands.

The diffraction phenomenon was first discovered by Italian scientist Grimaldi in the year 1665 and was correctly interpreted by Fresnel. He combined the Huygens’ principle of secondary wavelets with the principle of interference and concluded that diffraction occurs due to the interference of secondary wavelets originating from various points of the wavefront which are not obstructed by the obstacle. The diffraction phenomenon occurs only when a part of the advancing wavefront is obstructed by some sharp obstacle.

Classification of Diffraction

The diffraction phenomenon is broadly classified into the following two general classes.

i. Fresnel's diffraction

In this class of diffraction, the light source and the obstacle are separated by finite distance. Therefore, the wavefront incident on the obstacle is either cylindrical or spherical.

ii. Fraunhofer's diffraction

In this class of diffraction, the light source and the obstacle are separated by infinite distance. Therefore, the wavefront incident on the obstacle is plane. In the laboratory, infinite distance is created by placing a lens in between the light source and the obstacle so that the light source is at the focus of the lens.

Fresnel's Explanation of Rectilinear Propagation of Light

The greatest difficulty encountered by the supporters of the wave theory of light was how to explain the observed fact that light propagates in a straight line.