In a double slit experiment,when a thin film of thickness $t$ having refractive index $\mu$ is introduced in front of one of the slits,the maximum at the centre of the fringe pattern shifts by one fringe width. The value of $t$ is ($\lambda$ is the wavelength of the light used).

• A
  $\frac{\lambda}{2(\mu - 1)}$
• B
  $\frac{\lambda}{(\mu - 1)}$
• C
  $\frac{\lambda}{(2\mu - 1)}$
• D
  $\frac{2\lambda}{(\mu - 1)}$