When a parallel beam of monochromatic light of wavelength λ be incident on the grating.Each silt will diffract at all direction. If N be the no.line per inch of grating then (a+b)=1 inch/N=2.54/N cm which are the separated by a width (a+b). In the figure point P and Q, Q and R, R and S etc. Then (a+b) is known as grating element or grating space. Let a and b be the width of each silt and width of opaque portion. then (a+b) is known as grating element.If N be the no.of line per inch of the grating then grating element is given by If a be the width of each silt and b be the width of opaque portion. This surface is known as plane transmission grating. of the fin equal distant and parallel lined(a) with a diamond point on a optically plane glass plate.the space between two lines is transparent to light and lined portion is opaque to light. of parallel silt of equal width and sepration from one other by equal opaque space(b) made by ruling a large no. Thisshows that clear pattern will be observed only in the case of narrow slits.ĭiffraction gratingis an arrangement consisting no. This show that width of central maxima is directly proportional to the distance between the slits. Width of the central maximum $\beta $= 2y = 2λD/d If D is the distance between theslit and screen. If y is the distance from the point 0 to the first minima, then the width of the centralmaximum is 2y. Since the angular r width of the central maxima is 2$\theta $ = 2 λ/d Width of the central maxima is the distance between two first minimaįoe small angle sin $\theta $$\~\: \theta $ The intensitydistribution of the diffraction patterndue to the single slit as function of$\theta $as shown in figureįigure shows the intensity oft he secondary maxima goes on decreasing with increase in the value of $\theta $ The diffracted rays meets at point p 1so that wavelets coming fromPQ will have path difference of QRgiven by QR=QO 1 – PO 2 = dsinθįor the secondary minima at O 1, the path difference should be 2nλ/2įor the secondary maxima, the path difference should be equal to (2n +1) λ/2 The intensity of the secondary maxima is less than 5% the intensity at central maxima. the width of the central maxima isdouble of the secondarymaxima. The secondary maxima lies in between the secondary minima.Ĭ. The intensity of the secondary maxima decreases with the increase of the angle $\theta $ī. From the diffraction pattern we canobserved thatĪ. also, alternate bright and the dark bands are observed on the either side of the central spot O which are the secondary maxima and minima respectively. Since all the rays have been focused at theO the darkness id observed at the either side of the bright O. According to the rectilinear propagation of the light, a bright image is formed at the centre O. after passing through the lens l 2,it will focus on the screen ST. Diffraction at narrow wire, straightedge ,etc are the examples of fresnel diffractionĬonsider the parallel beam of light incident normally on the slit of width d. The incident wave front is spherical is cylindrical. No lenses are used to make the light beam parallel and convergent. In this type of diffraction, screen and the source or both are at the finite distance from the obstacle or aperture. The incident wavefront is plane wavefrontDiffraction at diffraction grating, singleslit, etc are the examples of fraunhofer diffraction. Lenses are used one to focus the light coming from infinity to the aperture and other to focus the diffracted light on the screen. In this type of diffraction, screen and source or both are at infinite distance from the obstacle or aperture. Diffraction is caused when the wave encounters with the diffracting object which is explained by Huygens Fresnel principle and principle of the superposition of waves which consider every particle on a wavefront as point source for the secondary wave and the displacement at any subsequent point is the sum of these secondary waves andalsothe amplitude being the sum of amplitude of the individual wave varying from zero to sum of individual amplitude giving series of maxima and minima. The process of bending of light around the edge of an aperture or obstacle is called diffraction of light.
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