Numerical approximations may be used, including the Fresnel and Fraunhofer approximations. Under the Fraunhofer conditions, the light curve (intensity vs position) is obtained by multiplying the multiple slit interference expression times the single slit diffraction expression. 2.8 Fraunhofer diffraction at a double slit Two rectangular slits parallel to one another and perpendicular to the plane of the paper, width of each slit is say � and width of the opaque portion is �, figure 2.3. 3.1. Interference and Diffraction 14.1 Superposition of Waves Consider a region in space where two or more waves pass through at the same time. In double Fraunhofer diffraction, calculate the fringe spacing on a screen 50 cm away from the slits, if they are illuminated with blue light (wavelength = 4800Å). FRAUNHOFFER DIFFRACTION DUE TO A DOUBLE SLIT - b - a - b - Width of each slit=b Distance between two slits=a Also, a+b=d 1stslit consists of imaginary points, which are distance apart. Description: The adjacent figur e represents a narrow slit AB of width ‘e’.. 14.12 (b) shows the graph for diffraction term and Fig 6(c) represents the resultant distribution. Diffraction patterns from different slits Fig. Thomas Young performed a celebrated experiment in 1803 demonstrating interference from two closely spaced slits. The light passing through the slit will converge by converging lens on screen which is at a distance 'D' from the slit. Augustin-Jean Fresnel did systematic studies and calculations of diffraction around 1815. Consider a plane wave front incidents on the slit of width 'd'. β α β ⎛⎞ = ⎜⎟ ⎝⎠. The diffracted intensity on the screen is very large along the direction of incident beam [i.e along OP0]. Applying the theory of interference on the wave amplitudes  at the two slits gives the resultant wave amplitude (R). The intensity at point P1 on the screen is obtained by applying the Fraunhofer diffraction theory at single slit and interference of diffracted waves from the two slits. Let the width of both the slits be equal and it is ‘, ’ and they are separated by opaque length ‘, ’. Here slit separation b=0.1 mm and slit width a=0.02 mm. 3: Fraunhofer diffraction … The ± sign indicates maxima on both sides with respect to central maximum. This phase difference can be estimated as follows: Draw a normal from A1 to B1Q. The path difference between the rays coming from corresponding points in the slits A1B1 and A2B2 can be found by drawing a normal from A1 to A2R. A monochromatic plane wave front of wave length ‘. Two slits of Laser diffraction compared to intensity diagrams The pattern formed by the interference and diffraction of coherent light is distinctly different for a single and double slit. 14.12. The intensity at point P 1 may be considered by applying the theory of Fraunhofer diffraction at a single slit. Single Slit Double Slit. The diffraction pattern of two slits of width a that are separated by a distance d is the interference pattern of two point sources separated by d multiplied by the diffraction pattern of a slit of width a. Your email address will not be published. The intensity distribution at different points on the screen can be explained in terms of path difference between the incident and diffracted rays as follows. The plane of the slits are perpendicular to plane of the paper. This lecture include complete description about principle Maxima, Minima and secondary Maxima of double slit. Electric field (compare with Lecture 11): 14.12(a)represents the graph for interference term, Fig. The multiple slit arrangement is presumed to be constructed from a number of identical slits, each of which provides light distributed according to the single slit diffraction expression. The combined effect results in missing of certain orders of interference maxima. Diffraction maxima and minima: If the path difference B1C = e sinθn = ± nλ, where n = 1, 2, 3… then θngives the directions of diffraction minima. The wavelets proceeding from all points in a slit along their direction are equivalent to a single wave of amplitude starting from the middle point of the slit, Where On the other hand if the path difference is odd multiples of  i.e., , then θn gives the directions of minima due to interference of the secondary waves from the two slits on both sides with respect to central maximum. Diffraction due to N-Slits (Grating): An arrangement consisting of large number of parallel slits of the same width and separated by equal opaque spaces is known as Diffraction grating.Gratings are constructed by ruling equidistant parallel lines on a … The irradiance at point P from a double slit is given by the product of the diffraction pattern from single slit and the interference pattern from a double slit II. The Fraunhofer diffraction equation is an approximation which can be applied when the diffracted wave is observed in the far field, and also when a lens is used to focus the diffracted light; in many instances, a simple analytical solution is available to the Fraunhofer equation – several of these are derived below. Theory: Light is made incident on arrangement of double slit. The plane of the slits are perpendicular to plane of the paper. 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