 Fabry @-@ Pérot thin @-@ film etalons are used in narrow bandpass filters capable of selecting a single spectral line for imaging ; for example , the H @-@ alpha line or the Ca @-@ K line of the Sun or stars . Fig . 10 shows an Extreme ultraviolet Imaging Telescope ( EIT ) image of the Sun at 195 Ångströms , corresponding to a spectral line of multiply @-@ ionized iron atoms . EIT used multilayer coated reflective mirrors that were coated with alternate layers of a light " spacer " element ( such as silicon ) , and a heavy " scatterer " element ( such as molybdenum ) . Approximately 100 layers of each type were placed on each mirror , with a thickness of around 10 nm each . The layer thicknesses were tightly controlled so that at the desired wavelength , reflected photons from each layer interfered constructively . 
