White light is passed through a Fabry-Perot interferometer in the arrangement shown in Fig...

White light is passed through a Fabry-Perot interferometer in the arrangement shown in Figure 1, where the detector is a spectroscope. A series of bright bands appear. When mercury light is simultaneously admitted into the spectroscope slit, 150 of the bright bands are seen to fall between the violet and green lines of mercury at 435.8 nm and 546.1 nm, respectively. What is the thickness of the etalon?

Figure 1