A thin glass plate of thickness is $\frac { 2500 } { 3 } \lambda$ ($\lambda$ is wavelength of light used) and refractive index $\mu = 1.5$ is inserted between one of the slits and the screen in Young's double slit experiment. At a point on the screen equidistant from the slits, the ratio of the intensities before and after the introduction of the glass plate is:\\
(1) $2 : 1$\\
(2) $1 : 4$\\
(3) $4 : 1$\\
(4) $4 : 3$