A bit of historical review is given for the concept of gravitational waves and detection experiments. In the presence of gravitational waves, it is frequently asked whether the laser light gets stretched and shrunk in the same way as the mirror distances in the interferometer so that gravitational waves cannot be measured. To answer this question, we have reviewed the interaction between light and gravitational waves carefully in both the transverse-traceless (TT) gauge and the proper detector or locally Lorentz frame. In the TT frame, the laser light directly interacts with gravitational waves, its frequency gets modified, but mirrors remain at rest to the linear order of a gravitational wave perturbation. In the detector frame, on the other hand, the frequency of light does not change while “distances” between the beam splitter and the mirrors get modified. Recent results for gravitational wave observations up to the first half of the third observation run in the advanced LIGO and the advanced Virgo are briefly summarized. Tests of general relativity based on gravitational wave data observed during O1 and O2 are described. These tests include residual and inspiral-merger-ringdown signal consistency tests, parameterized deviations in the waveform model, and constraint on the speed of gravitational waves by comparisons with the propagation of the electro-magnetic counterpart observed in the binary neutron star merger event. Some interesting events observed during O3a are also discussed.