In XRD scan on Cu$,$ which has FCC structure, a peak showed up at $2$theta $=43\mathrm{.}2$deg $.$ If the interplanar spacing $($nm$)$ of this peak $($dhkl$)$ is $0\mathrm{.}209$ nm$,$: A$)$determine the wavelength of X$-$ray $($nm$)$ when the order of reflection is $1.$ B$)$What is the planar density $($mmol$/$m$2)$ of $(001)$ plane of Cu$?$