Given, $ \lambda =5000\,\,\overset{\text{o}}{\mathop{\text{A}}}\,=5\times {{10}^{-7}}m. $ $ \beta =5mm=5\times {{10}^{-3}}m $ . $ D=1\,\,m $ . $ d=? $ Fringe width $ \beta =\frac{D\lambda }{d} $ $ \therefore $ $ d=\frac{D\lambda }{\beta }=\frac{1\times 5\times {{10}^{-7}}}{5\times {{10}^{-3}}} $ $ =1\times {{10}^{-4}}m=0.1\,\,mm $