Exp. (b)Spectrometer is an instrument which is used to obtain a pure spectrum of white light. It is used to determine the wavelength of different colours of white light (or wavelength of monochromatic light source), the refractive index of the material of the prism and the dispersive power of the material of the prism.Pyrometer is infrared sensitive device, so it is used to detect infrared radiations.Nanometre is the small unit of distance and is not a device. Photometer is used to measure luminous intensity, illuminance and other photometric quantities.