If prism angle $\mathrm{\alpha }=1^{\circ }, \mathrm{\mu }=1.54$ distance between screen and prism $\left(\mathrm{b}\right)=0.7\mathrm{m}$ distance between prism and source $\mathrm{a}=0.3\mathrm{m}, \mathrm{\lambda }=180\mathrm{\pi } \mathrm{nm}$ then in Fresnal biprism find the value of $\mathrm{\beta }$ (fringe width) 

• A

  ${10}^{-4}\mathrm{m}$

• B

  ${10}^{-3}\mathrm{mm}$

• C

  ${10}^{-4}\times \mathrm{\pi m}$

• D

  $\mathrm{\pi }\times {10}^{-3}\mathrm{m}$