Question

Question #2205b

Answer 1

elektroarsenal.net

The unit cell of face centered cubic lattice (FCC) has been shown in above figure. It is obvious from the figure that `4R` is the length of the diagonal of every square face of the cell having length of each side `a`,the edge length of unit cube.
So using Pythagorean theorem we can write

`a^2+a^2=(4R)^2`

`=>16R^2=2a^2`
`=>R=a/(2sqrt2)`