On introducing a thin mica sheet of thickness $2\times {10}^{-6}m$ and refractive index 1.5 in the path of the waves, central bright maxima shifts by 'n' fringes.  Wavelength of wave used is $5000A^0$ , then 'n' is

The given values are

Thickness $t=2\times {10}^{-6}m$

Refractive index $\mu =1.5$

Wavelength $\lambda =5000A^0=5000\times {10}^{-10}m$

 Number of fringes $n=\frac{\left(\mu -1\right)t}{\lambda }$

                                      $=\frac{0.5\times 2\times {10}^{-6}}{5\times {10}^{-7}}=2$

                                $\therefore n=2$