The LIGO interferometer is used to detect the very very small compression and expansion of space as a gravity wave passes by. It is essentially a Michelson interferometer, each of whose arms L1 and L2 is 4 km long. The input is a 200-W laser at a wavelength of 1064 nm. Assume that at the start the interferometer arms are adjusted so that all the light exits to the detector. L1 = L2 gives the full 200 W coming to the detector. What is the minimum increase in L2 required so that only 100 W goes to the detector?