# Energy spectrum confusion

From the book titled The Concepts of Modern Physics by the author Arthur Beiser, a paragraph from the chapter Atomic Structure says-

"The experiment shows that atoms exhibit line spectra in both emission and absorption."

But it also says in the beginning of the text that:

"a photon is emitted only when electron jumps from higher energy to lower energy."

We know when atoms jumps to lower energy it emits energy. So I find the two sentences contradictory. Anybody please explain it.

## 2 Answers

But it also says in the beginning of the text that a photon is emitted only when electron jumps from higher energy to lower energy. We know when atoms jumps to lower energy it emits energy.

That forms your emission spectrum. Atoms only emit light of certain energies corresponding to the differences in the energy levels.

But the opposite is true. Using your wording:

A photon is absorbed only when it causes the electron to jump from lower energy to a higher energy. We know when atoms jumps to higher energy it needs to absorb energy.

That forms your absorption spectrum. Atoms only absorb light of certain energies corresponding to the differences in the energy levels. Atoms can both emit and absorb photons, as schematised above.

Emission occurs when an electron falls back from a higher energy level, designated here as $$E_1$$, to a lower one, here designated as $$E_2$$. The photon emitted then has energy:

$$E_1-E_2=hf$$

where $$h$$ is Planck's constant and $$f$$ the photon's frequency.

Absorption is almost literally the reverse: a photon is absorbed with energy:

$$E_1-E_2=hf$$

Most atoms have multiple $$(E_1,E_2)$$ combinations, resulting in multi-line emission/absorption spectra.