A laser of wavelength 1524.8 nm is split using a beamsplitter into two similar power beams. Using mirrors, the beams are aligned to approximately co-propagate with the same polarisation and they cross each other with a small angle of 23.66 degrees. In the crossing region transverse to the beams there is an optical standing wave (optical lattice) - what is the distance between successive bright (or dark) intensity zones in μm? [Cross-polarised beams have no standing wave. 0.3% Accuracy needed.]