Answer
$L=3.3mm$
Work Step by Step
We know that;
$L=Nd$
But,
$N=\frac{\lambda_{avg}}{m\Delta \lambda}$
Hence,
$L=\frac{\lambda_{avg}d}{m\Delta \lambda}$
We plug in the known values to obtain:
$L=\frac{589.3(10)}{3(589.59-589.00)}=3.3\times 10^3\mu m=3.3mm$
