A Fabry-Perot device contains two parallel part-reflective surfaces, each with a power re- flection coefficient R = 0.9, separated by an air gap. The device is illuminated by laser light of wavelength 589 nm, spread out at varying angles of incidence by a lens. The transmitted intensity versus angle is observed to show a series of concentric thin bright circles sepa- rated by dark regions. It is measured that the smallest (first) bright circle is at angle 4.37 degrees to the device axis, and the 8th bright circle (counting outwards) is at angle 14.61 degrees to the normal. Calculate the width of the air gap.