7. Assume that a gravitational wave causes a gravitational strain h = AL/L of 10-21 in the arms of a gravitational wave detector like the one pictured below. Assume that the interferometer is set to null (no detected power) in the absence of the gravitational strain. Calculate the phase difference of the light (488 nm) arriving at the detector from the two arms of the interferometer due to the strain, and use this result to estimate the detected power if the laser output Po is 10 W. Assume that the nominal arm length is L = 4 km and that the light makes an average of 50 round-trips in each arm of the interferometer. Quadrupole gravity wave Mirror Fabry-Perot cavity Arm 1 Mirror Fabry-Perot Mirror cavity P1 = Po/4 Arm 2 Mirror Beam P2= Po/4 splitter Photodetector Po Laser