 The arm "tunnels" house vacuum pipes with a 120 cm diameter in which the laser beams are travelling under ultra-high vacuum.To increase the interaction between the light and an incoming gravitational wave, a Fabry-Perot optical cavity is installed in each arm as well as a mirror called "recycling mirror" at the instrument entrance, between the laser source and the beam splitter.Virgo is sensitive to gravitational waves in a wide frequency range, from 10 Hz to 10,000 Hz.