Hindawi Publishing Corporation Advances in Optical Technologies Volume 2010, Article ID 613728, 7 pages doi:10.1155/2010/613728 Research Article Spectral Modification by Diffraction and Scattering M. Taghi Tavassoly, 1 Masoomeh Dashtdar, 2 and Mohammad Amiri 3 1 Physics Department, University of Tehran, 14395-547 Tehran, Iran 2 Department of Physics, Shahid Beheshti University, Evin, 1983963113 Tehran, Iran 3 Physics Department, Bu-Ali Sina University, 65178 Hamedan, Iran Correspondence should be addressed to M. Taghi Tavassoly, tavasoli@iasbs.ac.ir Received 1 December 2009; Accepted 2 March 2010 Academic Editor: Oleg V. Angelsky Copyright © 2010 M. Taghi Tavassoly et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Different effects induce spectral changes, for example, correlation of source fluctuations, propagation of light, random changes in optical properties of a medium, diffraction and scattering from objects, and rough interfaces. We review the spectral changes that occur as a result of light diffraction from phase steps, and particularly we discuss the spectral changes in the neighborhood of phase singularities. We also review the redshift and blueshift in the spectra of the lights coherently and diffusely scattered from rough interfaces. In addition, we study the effects of roughness and incident angle on the spectral profiles of scattered lights in reflection and transmission modes. 1. Introduction After being realized in 1986 [1] that the spectrum of radiation may change on propagation, a considerable volume of works has been carried out dealing with spectral changes induced by the correlated fluctuations in sources and changes in coherence properties of radiations [2, 3]. Since the spatial coherence width of a light beam increases with the distance from the source, any light beam is coherent to some extent that depends on the source size and the distance from the source. Thus, as a coherent or partially coherent polychromatic beam of light passes through a medium that imposes different phases on different parts of the beam, the beam scatters from these parts and interference of the scattered lights leads to spectral changes. In this paper we study the spectral changes that occur as a result of light diffraction from phase steps and light scattering from rough interfaces in reflection and transmission modes. 2. Modified Spectrum of Diffracted Light When a fully or partially coherent beam of light experiences a sharp change in amplitude or in phase, redistribution of intensity occurs in space that is called diffraction. A sharp change in amplitude occurs as the beam passage is partly obstructed by an opaque object. A sharp change in phase happens as a beam of light is reflected from a step, Figure 1(a), or passes through a transparent plate immersed in a liquid or gas where a sharp change in refractive index is imposed at the plate edge, Figure 1(b). The lights diffracted from the neighborhood of the change-affected area interfere and lead to different spectra at different points in the diffraction field. The amount of spectral change varies from point to point, but it is remarkable and anomalous in the neighborhoods of phase singularities. Gbur et al. [4] used the Fresnel-Kirchhoff integral and obtained the following expression: M(r , λ) = 1 λ       W e ikR R d 2 r ′      2 (1) for spectral modifying function for a fully coherent spher- ical wave front passing through a lens, at points in the neighborhood of the geometrical focal point. In (1), r , λ, k, d 2 r ′ , and R stand for the distance between the focal point and observation point, wavelength, wavenumber, surface element, and the distance between a point on the aperture and observation point. The above function leads to drastic spectral changes in the neighborhoods of the phase singularities on the symmetry axis. But intensities at these