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Recent experiments on ultra slow light in strongly dispersive media byseveral research groups reporting slowing down of the optical pulses down tospeeds of a few metres per second encourage us to examine the intriguingpossibility of detecting a deflection or fall of the ultra slow light underEarth's gravity, i.e., on the laboratory length scale. In the absence of ausable general relativistic theory of light waves propagating in such astrongly dispersive optical medium in the presence of a gravitational field, wepresent a geometrical optics based derivation that combines {\it the effectivegravitational refractive index} additively with the usual optical dispersion.It gives a deflection, or the vertical fall $\Delta$ for a horizontal traversal$L$ as \[ \Delta = \frac{L^2}{2}\big(\frac{R_{\oplus G}}{R_\oplus^2}\big) n_g\big(\frac{1}{1+n_g\frac{R_{\oplus G}}{R_\oplus}}\big), \] where $R_{\oplusG}/R_\oplus$ is the ratio of the gravitational Earth radius($R_{\oplus G}$) toits geometrical radius $R_\oplus$, and $n_g$ is the group refractive index ofthe strongly dispersive optical medium. The expression is essentailly that forthe Newtonian fall of an object projected horizontally with the group speed$v_g=c/n_g$, and is tunable refractively through the index $n_g$. For $L \sim 1m$ and $n_g = c/v_g \sim 10^8$ (corresponding to the ultra-slow pulse speed$\sim few \times 1 ms^{-1}$), we obtain a fall $\Delta \sim 1 \mu m$, thatshould be measurable $-$ in particular through its sensitive dependence on thefrequency that tunes $n_g$.

Deflection of ultra slow light under gravity