Answer
$\lambda = 667~nm$
Work Step by Step
We can find the wavelength of the light:
$w = \frac{2~\lambda~L}{a}$
$\lambda = \frac{w~a}{2~L}$
$\lambda = \frac{(0.010~m)(0.20\times 10^{-3}~m)}{(2)~(1.5~m)}$
$\lambda = 6.67\times 10^{-7}~m$
$\lambda = 667~nm$
