A planar light with a wavelength of 500 nm is dropped on two long slits with a width of 0.10 mm with an interval of 0.20 mm on a light-proof screen. If the curtain is 1m away from the slit and 0.5 m away from the Weld slit, does the resulting pattern correspond to Fraunhofer diffraction or Fresnel diffraction? How many interference fringes are visible in the bright central strip.

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Bryant Liu · Accepted Answer

In the Fresnel diffraction the source of light and screen are placed at a finite distance. Given that,  λ  =  500   nmd=0.20 mmD= 1 mx=0.5 mAlso the value of slit width, a=0.10 mmThe formula to calculate the width of the central maxima is given by,  ω  =            2      λ      D        a  and the formula to calculate the fringe width in the Fresnel diffraction   β  =            λ      D        d  Therefore the number of fringes which can be accommodated n is   n  =      ω    β      n  =  2      d    a  Therefore substituting the given data into the above equation.  n  =            2      ×      0.20        0.10      n  =  4Since in the given problem the distance between the slit and screen is finite which is 1 m, therefore the given diffraction is Fresnel Diffraction. And the number of fringes which can be visible will be approximately 3, since the side fringes may not be completely bright.

How many interference fringes are visible in the bright central strip?

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