A transparent rectangular slab of length $l=20\text{ cm}$ and thickness $d=4.0\text{ cm}$  is placed in front of a converging lens of focal length  $f=20\text{ cm}$. A screen S placed in the focal plane of the lens. Refractive index of the material of the slab increases linearly from ${\mu }_0=1.5$  at the bottom face by an amount $\delta \mu =3.0\times {10}^{-4}$  between the bottom and the top faces. A parallel beam is made to incident on the slab as shown in the figure. The position on the screen where the parallel  beam of light converges is  $y=\_\_\_\_\_\_\_(\times {10}^{-2}cm)$   [ $d\mu <<\mu$ ]