Astronomers analyse and measure the wavelengths of lights reaching earth from different stars and similar objects to determine their chemical composition. Such light is collected by a telescope. The most common technique for analysing visible light is spectroscopy. This involves breaking up light into its individual colours of spectrum. Spectroscopic techniques are also used to break up other types of radiation into individual wavelengths.
Astronomers identify the types of atoms that make up the star's gaseous outer layers, and identify the molecules in the atmosphere of a planet by analysing the spectrum of light coming from those bodies.Astronomers use different types of spectrometers, One type measures the wavelengths in a spectrum. Another type, called a spectroscope, produces a spectrum to be viewed with the eye. A third type called spectrograph records the image of a spectrum on a photographic plate or some other device.
In a typical spectrometer light enters it through the narrow entrance slit and passes through a collimating lens. This lens causes the light to become a beam of parallel light rays. The parallel light then travels through a prism, where it is broken up into a spectrum. A lens focuses the light on the exit slit. Only one colour of light can pass through this slit at a time. Therefore, the prism must be rotated to bring the other colours into the exit slit and to scan the entire spectrum. A circular scale records the angle of the prism, from which the wavelength of the light can be determined.
Some spectrometers have a flat mirror called a grating, instead of a prism. The surface of a grating is lined with thousands of narrow, parallel grooves. Upon striking a grating, a parallel beam of light spreads out into a spectrum.