In a Young's double-slit experiment with wavelength $\lambda$,the fringe width is $\beta$. When two glass plates of thicknesses $t_1$ and $t_2$ $(t_1 > t_2)$ and refractive index $\mu$ are placed in the paths of the two light beams respectively,by what distance will the fringe pattern shift?

• A
  $\frac{\beta (\mu - 1)}{\lambda} (t_1 - t_2)$
• B
  $\frac{\beta (\mu - 1)}{\lambda} \left( \frac{t_1}{t_2} \right)$
• C
  $\frac{\mu \beta}{\lambda} \frac{t_1}{t_2}$
• D
  $(\mu - 1) \frac{\lambda}{\beta} (t_1 + t_2)$