Answer:
The thickness of the paper is  [tex]t = 188\mu m[/tex]
Explanation:
From the question we are told that
  The length of the wedge-shaped air film  [tex]L = 12.5 cm = \frac{12.5}{100} = 0.125m[/tex]
  The wavelength of the light is  [tex]\lambda = 600nm = 600* 10^{-9}m[/tex]
 The spacing of the interference fringe is  [tex]D = 0.200mm = \frac{0.200}{1000} = 0.2*10^{-3} m[/tex]
For destructive interference the thickness is mathematically represented as
        [tex]t =\frac{\lambda * L}{2 * D }[/tex]
Substituting values
        [tex]t = \frac{600 * 10^{-9} * 0.125 }{2 * 0.2 *10^{-3}}[/tex]
        [tex]t = 188\mu m[/tex]
