1 nm = 1 * 10^-9 m
1mm = 1* 10^-3 m = 10^6 nm.
Thus, 10^6 nm = 1 mm
=> 261 nm = 261 * 1/10^6 mm = 261 * 10^-6 mm = 2.61 * 10^-4 mm. :)
