Balmer Series: emission spectrum

Here is a graphic of an emission spectrum set-up and an emission spectrum. For an emission spectrum, the gas of atoms you wish to study is the source. Photons are emitted from the hot gas of atoms when electrons in the atoms drop down from a higher potential energy state to a lower potential energy state. Energy is conserved, and the electron energy is converted to photon energy in this transition. The photon is created in this process. It does not exist before this transition takes place; it does exist after the transition. The electron falls from a high energy (excited) state to a lower energy state; the energy it loses goes to creating the photon. These photons come out at very specific frequencies.

The role of the prism is to separate the photon beam into its different colors; before the prism all the photons are moving together in a single light beam. After the beam goes through the prism, whatever colors are present are separated, as shown.

The moving aperture allows only one frequency (color) to pass though at a time. As it moves it scans through the spectrum. The graph shows the Intensity measured at the detector as a function of frequency.

An absorption spectrum is the complement of this. The atoms are located in the beam and instead of emitting photons they absorb photons in a process opposite to that of emission. That is, the electron starts out in a low-energy state of the atom and goes to a higher energy state. A photon is anihilated (absorbed) in this process, thus energy is conserved via the equation E1 + hf = E2, where E1 is the energy of the state the electron starts in and E2 the energy of the state it goes to. hf is the energy the photon gives to the electron, after which the photon ceases to exist. (This is a "zero sum game". )

This leads to dark "absorption" lines, in an otherwise broad and featureless reference spectrum. These dark lines are due to the quantum absorption process described in the preceding paragraph. Using the energy it gets from the photon, the electron makes a quantum leap upward to a higher energy state.