Emission vs Absorption Spectra: How Atoms Reveal Themselves

Heat sodium in a flame and it glows a vivid yellow-orange. Shine white light through sodium vapour and a thin dark gap appears in the rainbow, at exactly the same yellow wavelength. These two observations are two sides of the same coin: an emission spectrum and an absorption spectrum. Both come from electrons jumping between fixed energy levels, and together they’re among the most powerful tools in science, used to identify elements in a lab sample or in a star billions of kilometres away.

Three kinds of spectrum

It helps to start with the three basic types, first described by the German physicist Gustav Kirchhoff around 1860:

Type	How it’s produced	What you see
Continuous spectrum	a hot, dense object: a solid, liquid or dense gas (e.g. a light-bulb filament, the Sun’s surface)	an unbroken rainbow of all colours
Emission (bright-line) spectrum	a hot, low-density gas	a few bright coloured lines on a dark background
Absorption (dark-line) spectrum	a cooler gas in front of a source of continuous light	a rainbow crossed by dark lines

Quick comparison: emission vs absorption

Feature	Emission spectrum	Absorption spectrum
Electron transition	high → low energy level	low → high energy level
Energy	released as photons	absorbed from incoming light
Appearance	bright lines on a dark background	dark lines on a bright continuous background
Needs	energy input to excite atoms (heat, electricity)	a continuous light source behind the sample
Line positions	characteristic of the element	the same wavelengths as its emission lines
Everyday examples	flame tests, neon signs, sodium lamps	dark lines in sunlight, coloured solutions

How emission spectra form

1. Energy (heat or electricity) excites electrons to higher energy levels.
2. Excited states are unstable; electrons fall back to lower levels.
3. Each fall releases a photon with energy equal to the gap: ΔE = hf = hc ÷ λ.
4. Because the levels are fixed, only certain wavelengths appear, as bright lines.

See photons and energy levels, flame test colours and the hydrogen emission spectrum.

How absorption spectra form

1. White light, containing all wavelengths, passes through a cooler gas.
2. Atoms absorb photons whose energies exactly match the gaps between their energy levels, lifting electrons to higher levels.
3. Those wavelengths are removed from the transmitted beam, leaving dark lines.
4. (The excited atoms do re-emit light, but in random directions, so little of it continues along the original beam.)

Why the lines match

The same energy gaps are involved in both processes. A sodium electron moving between two particular levels:

• emits a photon of 589 nm when falling down
• absorbs a photon of 589 nm when jumping up

So an element’s dark absorption lines appear at exactly the same wavelengths as its bright emission lines. This is why both kinds of spectra can identify the same element.

One subtlety: in a cool gas, most atoms are in their ground state, so absorption spectra mainly show transitions from the ground state. Emission spectra from a hot gas can include transitions between many different excited levels, so an emission spectrum often has more lines than the matching absorption spectrum.

Historical story: Fraunhofer lines and helium

In 1814, the German optician Joseph von Fraunhofer looked closely at sunlight through a prism and found it crossed by hundreds of fine dark lines. He labelled the strongest with letters (A, B, C, D…). He couldn’t explain them.

Around 1859, Robert Bunsen and Gustav Kirchhoff showed that the Fraunhofer D lines in the yellow match the bright yellow emission lines of sodium. The dark lines were absorption by sodium atoms in the Sun’s cooler outer layers. For the first time, scientists could determine the chemical composition of the Sun without ever visiting it.

Bunsen and Kirchhoff went on to discover two new elements from their emission spectra: caesium (1860, named from the Latin for sky blue, after its blue lines) and rubidium (1861, from the Latin for deep red).

In 1868, during a solar eclipse, astronomers observed a bright yellow emission line in the Sun’s outer atmosphere that didn’t match any known element. Norman Lockyer proposed a new element, naming it helium, after the Greek helios, the Sun. Helium wasn’t isolated on Earth until 1895. It’s the only element discovered in space before it was found on Earth. See noble gases.

Uses in chemistry

• Flame tests and flame emission identify metal ions by their emission colours.
• Atomic absorption spectroscopy (AAS) measures how much light of an element’s specific wavelength is absorbed by atoms in a flame, giving concentrations down to parts per billion. See atomic absorption spectroscopy.
• ICP-OES measures emission from atoms in a very hot plasma, analysing dozens of elements at once. See AAS vs ICP.
• UV-visible spectroscopy and colorimetry measure absorption by molecules and ions in solution. Molecules give broad absorption bands rather than sharp lines, because molecular energy levels include many closely spaced vibrational and rotational levels. See UV-vis spectroscopy.

Uses in astronomy

• Composition of stars and galaxies from their absorption lines.
• Temperature: the pattern and strength of lines depend on how many atoms are excited or ionised.
• Motion: lines shifted to longer wavelengths (redshift) show objects moving away; shifts to shorter wavelengths (blueshift) show approach. Redshifts of distant galaxies revealed that the universe is expanding.
• Nebulae glow with emission spectra, often dominated by hydrogen’s red H-alpha line, because their gas is excited by nearby hot stars.

A worked comparison: sodium

	Emission	Absorption
Set-up	sodium salt heated in a flame	white light through sodium vapour
Observation	bright yellow-orange lines at 589.0 and 589.6 nm	two dark lines at 589.0 and 589.6 nm in the yellow
Transition	3p → 3s	3s → 3p

Common misconceptions

• “Absorption lines are where the atom has no electrons.” They’re where the atom absorbs specific wavelengths.
• “Coloured flames show every wavelength the element absorbs.” Emission and absorption share line positions, but emission from hot gases usually shows more lines.
• “A solution’s colour is the colour it absorbs.” It’s the opposite: the solution transmits the colours it doesn’t absorb. See spectroscopy misconceptions.

A simple way to remember the difference: emission is emitted from a glowing gas; absorption is absorbed from a beam of light passing through a gas.

Key takeaways

• Continuous spectra come from hot dense objects; emission spectra from hot thin gases; absorption spectra from cool gas in front of a continuous source.
• Emission: electrons fall and release photons (bright lines). Absorption: electrons jump up by absorbing photons (dark lines).
• An element’s absorption and emission lines occur at the same wavelengths because they involve the same energy gaps.
• Fraunhofer’s dark lines in sunlight revealed the Sun’s composition and led to the discovery of helium.
• Emission and absorption underpin flame tests, AAS, ICP-OES and much of astronomy.