The frequency of the emitted photon is then derived from the Planck formula. Spectral lines from transitions between electron energy states in hydrogen. Figure 19.3 shows the possible transitions between the lowest four energy levels of hydrogen plus the ionized state in which the electron is initially a large distance from the hydrogen nucleus. Transitions from any state to the ground state form a series called the Lyman series , while transitions to the first excited state are called the Balmer series , transitions to the second excited state are called the Paschen series , and so on. Within each series, increasing frequencies are labeled using the Greek alphabet, so the transition from to is called the Lyman- spectral line, etc.
Atoms can absorb as well as emit radiation.
For instance, if hydrogen atoms in the ground state are bombarded with photons of energy equal to the energy difference between the ground state and some excited state, some of the atoms will absorb these photons and undergo transitions to the excited state. Examination of the light with a spectroscope after it passes through a gas of atoms will show absorption lines where the photons with the critical energies have been removed. This is one of the main ways in which astrophysicists learn about the elemental constitution of stars and interstellar gases. Atoms in excited states emit photons spontaneously. However, a process called stimulated emission is also possible.