Let the thickness of the Glass slab is \\\\(t\\\\)The condition for minimum thickness corresponding to a dark band is \\\\(2\\mu t \\,cosr = \\lambda\\\\)\\\\( \ herefore t= \\dfrac{\\lambda}{2 \\mu cos \\,r} = \\dfrac{5890 \ imes 10^{-10}}{2 \ imes 1.5 \ imes cos 60^o} \\\\) \\\\(t=3926 \ imes 10^{-10} m = 3926 {\\overset{o}A} \\\\)