What is a use of examining light spectra?

One use of light spectra has an application in astronomy. When looking at distant objects like galaxies, you can tell if they are moving away from you or closer to you. If the spectral light appears blue (blue-shift) it means that the object is moving toward you, if the spectral light is red (red-shift) it means that the object is moving away from you. Another astronomy application is to look at the light from a star through a spectrum and by doing so you can figure out the chemical composition of the star just by the color of its spectrum.