A spectrograph is an instrument used to separate and measure the wavelengths present in Electromagnetic radiation and to measure the relative amounts of radiation at each wavelength. In other words obtain and record the spectral content of light or its 'spectrum'. The spectrograph splits or disperses the lightfrom an object into its component wavelengths so that it can be recorded then analysed. Light entering a spectrograph can be split or dispersed into a spectrum by one of two means, using a prism or a diffraction grating.
A diffraction grating is an optical element, which separates (disperses) polychromatic light into its constituent wavelengths(colors). The polychromatic light incident on the grating is dispersed so that each wavelength is reflected from the grating at a slightly different angle. The dispersion arises from the wavefront division and interference ofthe incident radiation from the periodic structure ofthe grating. The dispersed light is then re-imaged by the spectrograph and the required wavelength range is directed to a detection system. Gratings consist of equally spaced parallel grooves, formed on a reflective coating and deposited on a substrate.