Answer
$3.1\times10^{-16}m$
Work Step by Step
Calculate the time for the light to cross the elevator horizontally.
$$\Delta t = \frac{\Delta x}{c}$$
How far the elevator moves vertically during that time is the beam’s apparent deflection.
$$\Delta y = \frac{1}{2}g(\Delta t)^2=\frac{g(\Delta x)^2}{2c^2}$$
$$=\frac{(9.80m/s^2)(2.4m)^2}{2(3.00\times10^8m/s)^2}$$
$$=3.1\times10^{-16}m$$
This is smaller than an atomic nucleus.
