Emission line spectrum কী?

Example 2.11 (Hydrogen and sodium)

Excited hydrogen emits exactly four lines in the visible: a red line at $656\,\mathrm{nm}$, a blue-green line at $486\,\mathrm{nm}$, and two violet lines at $434\,\mathrm{nm}$ and $410\,\mathrm{nm}$. Excited sodium emits essentially one strong yellow-orange line at $589\,\mathrm{nm}$ — which is why older street lamps bathe whole neighborhoods in that single color: their light contains almost nothing else.