An emission spectrum is the type of light a particular substance emits. Every element gives off a unique fingerprint of light. Analyzing the frequencies of this light helps identify the chemical that generated it. This process is called emission spectroscopy and is a very useful scientific tool. A prism that splits light into its different colors is a basic form of a spectrometer, the instrument of spectroscopy. In astronomy, spectrometers are used to reveal a star or planet’s emission spectrum. From this, it is possible to determine the object’s composition.

Light is actually electromagnetic radiation with a specific frequency range. This is commonly referred to as the range of visible light. Blue light, for example, has a higher frequency than red light. Frequency is proportional to how energetic the radiation is, so blue light is also more energetic than red light. All elements and compounds emit some radiation, but this is not necessarily in the frequency range of visible light. Very hot substances usually do emit visible light, which explains why we can see the light from the sun.

The emitting substance in an emission spectrum can be a pure element, such as iron. It also could be a chemical compound such as water. Atoms and molecules tend to transition between higher and lower energy states. In an atom this might mean that an electron jumps from a higher orbit to a level closer to the nucleus of the atom. This would lower its energy state. In a molecule there can be changes in rotational or vibrational energy. In general, the higher the energy state, the more capable an atom or molecule is of emitting light.

Any lost energy in an atom or molecule must go somewhere. It can be emitted in the form of a photon, the basic unit of light. Photons can only occur in whole numbers; it’s not possible to have fractions of photons. This release of photons from atoms and molecules gives rise to an emission spectrum.

It is important to distinguish an emission spectrum from an absorption spectrum. In an absorption spectrum, some wavelengths of emitted light are absorbed as they passes through a gas. This can be observed when light from the sun passes through the atmosphere of Venus before reaching telescopes on Earth. When compared with the sun’s regular emission spectrum, an absorption spectrum can help reveal the composition of Venus’s atmosphere.