In a Young's double slit experiment, each of the two slits $A$ and $B$, as shown in the figure, are oscillating about their fixed center and with a mean separation of 0.8 mm . The distance between the slits at time $t$ is given by $d=(0.8+0.04 \sin \omega t) \mathrm{mm}$, where $\omega=0.08 \mathrm{rads}^{-1}$. The distance of the screen from the slits is 1 m and the wavelength of the light used to illuminate the slits is $6000 A$. The interference pattern on the screen changes with time, while the central bright fringe (zeroth fringe) remains fixed at point $O$. The maximum speed in $\mu \mathrm{m} / \mathrm{s}$ at which the $8^{\text {th }}$ bright fringe will move is.