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1. 1. DOUBLE SLIT DIFFRACTION PRESENTED BY SARA DEPARTMENT OF PHYSICS 3RD SEMESTER (B.S)
2. 2. DIFFRACTION: The bending of wave around an obstacle is called diffraction. APPLICATIONS: Because of diffraction we can hear sound outside the close room. 2
3. 3. INTRODUCTION: The double-slit experiment is a nineteenth-century investigation into the properties of light. This experiment is sometimes referred to as Young's experiment. The experiment belongs to a general class of "double path" experiments, in which a wave is split into two separate waves that later combine back into a single wave. 3
4. 4. DOUBLE SLIT DIFFRACTION: Young began his experiment by sending waves of coherent light towards a barrier with two slits. The two slits are separated by a distance d. On a screen, a perpendicular distance L from the slits, a series of interference fringes were viewed. The formulas we will now develop will allow us to determine if a point P in the interference pattern a distance y from its center will fall into a bright zone or into a dark zone. 4
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7. 7. EXPLAINATION: EPD represents extreme path difference, or the effective difference in the distances the light must travel to reach a given position on the screen from each of the slits. For bright fringes, this path difference must be a multiple of a wavelength to insure constructive interference. When this overall path difference equals an odd-multiple of a half wavelength, then destructive interference is insured and a dark fringe is formed on the screen. 7
8. 8. CONDITIONS: Thomas Young's equation for double slit interference is For bright fringes (constructive interference) For dark fringes (destructive interference) 8 where L y d EPD ' sin L y tan mEPD ........)3,2,1,0(m 2 )12( mEPD ........)3,2,1(m
9. 9. EXAMPLE OF LASER LIGHT: 9
10. 10. 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. The single slit intensity envelope is shown by the dashed line and that of the double slit for a particular wavelength and slit width is shown by the solid line. The photographs of the single and double slit patterns produced by a helium-neon laser show the qualitative differences between the patterns produced 10
11. 11. LASER DIFFRACTION COMPARED TO INTENSITY DIAGRAMS The number of bright maxima within the central maximum of the single-slit pattern is influenced by the width of the slit and the separation of the double slits. 11
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