IB Physics HLTopic 5 — Atomic & NuclearPaper 1 & 2discrete energy levels~15 min read
Emission & Absorption Spectra
Heat a gas and it glows — but not with every colour. Split that glow through a prism and you don’t get a rainbow; you get a handful of razor-sharp coloured lines on a black background, always at exactly the same wavelengths. Shine white light through a cool version of the same gas and those exact colours go missing, leaving dark lines. These spectra are the clinching evidence that electrons in atoms can only sit at discrete energy levels — and that every element has its own unique fingerprint of light.
📘 What you need to know
Atomic spectra appear when atoms emit or absorb light of specific wavelengths
They are evidence that electrons can only exist at discrete (quantised) energy levels
Emission spectrum: bright coloured lines on a black background, from a hot gas
Emission happens when an excited electron drops to a lower level and emits a photon
Absorption spectrum:dark lines on a continuous coloured background, from a cool gas
Absorption happens when an electron absorbs a photon of exactly the right energy and jumps up
The dark lines sit at the same wavelengths as the bright lines for the same element
Each element produces a unique pattern of lines, so spectra can identify elements
Each line corresponds to a specific transition between two energy levels
Emission spectra
Heat a low-pressure gas (or pass an electric discharge through it) and you give its electrons energy. They jump up to higher levels — but they don’t stay there. As each electron falls back down, it releases the energy it borrowed as a single photon of a very specific wavelength.
Heating excites electrons to higher energy levels
When an electron drops to a lower level, it emits a photon
Each transition gives a specific wavelength — one bright spectral line
The result: discrete coloured lines on a black background
The visible hydrogen emission spectrum: four bright lines on black, each one a photon released by an electron dropping between two specific energy levels.
Absorption spectra
Now do the reverse. Shine white light (which contains all wavelengths) through a cool low-pressure gas. Electrons in the gas absorb only the photons whose energy exactly matches a jump between two levels — and those wavelengths are removed from the light that comes out.
White light passes through a cool gas
Only photons with exactly the right energy are absorbed, exciting electrons up
Those wavelengths are missing from the transmitted light — leaving dark lines
The result: dark lines on a continuous coloured background
The absorption spectrum is the negative of the emission spectrum: dark lines fall at exactly the same wavelengths where the emission spectrum had bright ones.
Here’s the beautiful symmetry. Emission and absorption are the same jumps run in opposite directions. A photon of 656 nm is released when an electron falls between two particular levels, and a photon of 656 nm is swallowed when an electron climbs between the very same two levels. Same energy gap, same wavelength — that’s why the dark lines and bright lines line up perfectly for any given element.
Electron drops
emits photon
Bright line (emission)
same gap, reversed
Dark line (absorption)
Emission spectrum
Absorption spectrum
Source
Hot, low-pressure gas
White light through a cool gas
Electron moves
Down (emits photon)
Up (absorbs photon)
Appearance
Bright lines on black
Dark lines on a rainbow
Line positions
Identical for the same element
A fingerprint for every element
Because the energy levels are unique to each element, so is its spectrum. No two elements produce the same set of lines. This lets us identify what a substance — even a distant star — is made of, just by looking at the light it emits or absorbs.
Hydrogen shows a strong red line at 656 nm
Sodium burns with a characteristic yellow flame (strong line at 589 nm) — used in street lights
Mercury’s lines are mostly below 450 nm, giving a blue light
Neon glows in the red-orange, hence its use in colourful signs
This is one of the most powerful ideas in physics: we can read the composition of a star we will never visit just by splitting its light. The dark absorption lines in sunlight told astronomers what the Sun is made of — in fact helium was discovered in the Sun’s spectrum before it was ever found on Earth. Spectra are the universe’s barcode.
WE 1
A hot gas produces an emission spectrum of several bright lines on a black background. (a) Explain how these lines are produced. (b) State what the discrete (line) nature of the spectrum tells us about the atom.
(a) how the bright lines form
Electrons are excited to higher energy levels.
When they drop back down, they emit photons.
Each transition gives a photon of a specific wavelength → one line.
each line is a photon emitted as an electron falls between two levels(b) what the line spectrum shows
Only certain wavelengths appear, so only certain energy jumps are possible.
the atom’s energy levels are discrete (quantised)The key logic: discrete lines mean discrete energies, which mean discrete energy levels. If electrons could have any energy, you’d see a continuous smear of colour, not sharp separate lines.
WE 2
White light is passed through a cool cloud of hydrogen gas and then split into a spectrum. (a) Describe the appearance of the resulting spectrum. (b) State how it relates to the emission spectrum of hydrogen. (c) Explain why only certain wavelengths are absorbed.
(a) appearancea continuous coloured spectrum crossed by dark lines(b) relation to emission
The dark lines are at the same wavelengths as the bright emission lines.
it is the “negative” of the emission spectrum for the same element(c) why only certain wavelengths?
Only photons whose energy exactly matches an energy-level gap can be absorbed.
only photons matching a transition energy are absorbed; the rest pass throughThe gas is picky: it can only absorb a photon if that photon carries precisely the right energy to lift an electron between two of its levels. Every other wavelength sails straight through, which is why the background stays a full rainbow.
WE 3
Two unknown gases are analysed. Gas A shows bright lines at 486 nm and 656 nm; gas B shows bright lines at 447 nm and 588 nm. (a) State whether A and B are the same element. (b) Explain how astronomers can use spectra to identify the elements in a distant star.
(a) same element?
The two gases produce different sets of lines.
no — different line patterns mean different elements(b) identifying a star’s elements
Each element has a unique pattern of spectral lines.
Split the star’s light and match the line positions to known elements.
compare the star’s spectral lines with the known fingerprint of each elementGas A’s 486 nm and 656 nm are actually two of hydrogen’s signature lines. Because no two elements share a line pattern, matching the wavelengths is like matching a barcode — it uniquely names the element, even light-years away.
🔬 Explaining a spectrum
Emission? Bright lines on black — electrons fall and emit photons.
Absorption? Dark lines on a rainbow — electrons absorb photons and jump up.
Why lines, not a smear? Energy levels are discrete, so only certain energies appear.
Why do positions match? Same energy gaps, run in opposite directions.
Identify an element? Match its unique line pattern to a known one.
Link to energy: each line = one transition = one photon energy.
💡 Top tips
Emission = bright lines on black (electron drops, emits). Absorption = dark lines on colour (electron rises, absorbs).
The dark and bright lines are at the same wavelengths for the same element.
Discrete lines are evidence for discrete energy levels.
Each element’s spectrum is unique — a fingerprint for identification.
Only photons with exactly the right energy are absorbed or emitted.
Each line corresponds to one transition between two levels.
⚠ Common mistakes
Mixing up the two: emission is bright-on-black, absorption is dark-on-colour
Saying the background of an emission spectrum is coloured — it’s black
Thinking absorption removes random wavelengths — only exact-match photons are absorbed
Forgetting the dark and bright lines line up for the same element
Claiming a continuous spectrum shows energy levels — it’s the discrete lines that do
Saying electrons are “destroyed” or “created” — they simply move between levels
Quick recap:Emission spectra (bright lines on black) form when excited electrons drop to lower levels and emit photons; absorption spectra (dark lines on a rainbow) form when electrons absorb matching photons and jump up. For any element the two sets of lines sit at identical wavelengths, because they’re the same energy gaps reversed. The fact that spectra are made of discrete lines proves atoms have discrete energy levels, and because each element’s pattern is unique, spectra let us identify elements — even in distant stars.
Every one of these spectral lines is a single photon, and its colour is set entirely by its energy. To turn a wavelength into an energy — or work out which energy-level jump produced a given line — we need the photon model and one compact equation, E = hf. Next page: Photon Energy.
Spectra not quite clicking?
Book a free meeting and we’ll sort emission vs absorption, why the lines line up, and what discrete lines really tell you.
