ocr: SPECTRAL LINE Ac distinct line differing from the spectrum around it. It can be darker (ABSORP- TION LINE) or lighter (EMISSION LINE) than its surroundings. Spectral lines allow one to determine the presence or absence of different ele- ments. In astronomy spectral lines allow one also to calculate the speed at which dis- tant objects are refracting or approaching. When radiation of any wavelength passes through a cooler gas, a part of the photons in the radiation collide with the atoms or molecules of the gas, and tunes them to higher energy levels. Since only photons of certain wavelengths a ...
