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):
(1) $\frac{\lambda}{2(\mu-1)}$
(2) $\frac{\lambda}{(2\mu-1)}$
(3) $\frac{2\lambda}{(\mu-1)}$
(4) $\frac{\lambda}{(\mu-1)}$