IB Physics SLTopic 5 — The Atomic & Nuclear WorldPaper 1 & 2spectral lines = fingerprints~7 min read
Emission & Absorption Spectra
Split the light from a glowing gas through a prism and you don’t get a smooth rainbow — you get a set of sharp, separate coloured lines, like a barcode. Every element writes its own barcode in light, and it never changes. That’s how we know what distant stars are made of without ever visiting them.
📘 What you need to know
Atoms only emit or absorb light at certain wavelengths, giving atomic spectra made of discrete lines
An emission spectrum is bright coloured lines on a black background — made when excited electrons drop to lower levels and give out photons
An absorption spectrum is dark lines on a continuous coloured background — made when electrons absorb specific photons from white light
For the same element, the absorption lines sit at exactly the same wavelengths as the emission lines — one is the inverse of the other
The lines are discrete, not continuous — direct evidence that electron energy levels are discrete (quantised)
Each element has a unique pattern of lines — a fingerprint used to identify which elements are present (for example, in stars)
Emission Spectra
Heat a low-pressure gas (or run an electrical discharge through it) and you give its electrons energy, kicking them up to higher energy levels. They don’t stay there — each electron soon falls back down, and every time it drops it releases the lost energy as a photon of one specific wavelength. Collect all that light and you see a handful of bright coloured lines on a black background, one line per possible drop. Hydrogen, for example, shows four clear lines in the visible range.
Absorption Spectra
Now do the opposite: shine white light (which contains every wavelength) through a cool gas. The electrons snatch out only the photons whose energy exactly matches a jump up between their levels, so those specific wavelengths go missing. The light coming out the far side is a full rainbow with dark lines punched into it — and those gaps line up precisely with the bright lines the same gas would emit.
Hydrogen’s four visible lines appear bright on black in emission, and as dark gaps in the rainbow in absorption — at identical wavelengths. Bright lines and dark lines are two views of the same set of electron jumps.
emission: electron falls
→ emits photon →
bright line on black
absorption: electron jumps up
→ absorbs photon →
dark line in the rainbow
Why Lines, Not a Smooth Rainbow?
Here’s the deep point. If electrons could take any energy at all, atoms would emit and absorb every wavelength and we’d just see a smooth band. Instead we see sharp separate lines — which can only happen if electrons are restricted to a set of discrete energy levels. Each line matches one particular jump between two levels, and only those exact jumps are allowed. The spectra are the visible proof that energy inside an atom is quantised.
A Fingerprint for Every Element
Because every element has its own arrangement of energy levels, every element produces its own unique pattern of spectral lines — no two elements share the same set. That makes a spectrum as good as a fingerprint. Match the lines in starlight to known patterns and you can read off exactly which elements a star contains. The same physics lights our world: sodium street lamps glow yellow (a strong line at 589 nm), mercury lamps look bluish, and neon signs shine red — each colour the signature of its atoms’ transitions.
Three elements, three completely different line patterns. Hydrogen’s four lines, helium’s scattered six, and sodium’s dominant yellow line each identify their element on sight — the basis for reading the composition of stars.
Quick recap: exciting a gas gives an emission spectrum (bright lines on black); white light through a cool gas gives an absorption spectrum (dark lines on a rainbow) at the same wavelengths. Discrete lines prove energy levels are quantised, and each element’s unique pattern is a fingerprint for identifying it.
🧭 Emission vs absorption at a glance
Emission = bright coloured lines on black; made by exciting a gas (heat or discharge); electrons fall and emit photons
Absorption = dark lines on a continuous rainbow; made by white light through a cool gas; electrons absorb photons
Same element → the lines line up at the same wavelengths (one is the inverse of the other)
Discrete lines → electron energy levels are discrete / quantised
Unique pattern → identifies the element (a spectral fingerprint)
WE 1
(a) Describe how the emission spectrum of a low-pressure gas is produced. (b) State how an absorption spectrum of the same gas differs in appearance.
Part (a) — emission
The gas is heated / given an electrical discharge, exciting electrons to higher energy levels
As each electron falls back down it emits a photon of a specific wavelength
→ bright coloured lines on a black backgroundPart (b) — absorption
White light through the cool gas has those same wavelengths removed
→ dark lines on a continuous (rainbow) background, at the same wavelengths
WE 2
(a) Explain why an atomic spectrum consists of discrete lines rather than a continuous band of colour. (b) State how spectra can be used to identify the elements present in a distant star.
Part (a) — why discrete lines
Electrons can only occupy discrete energy levels
So a transition releases (or absorbs) only a specific photon energy → a specific wavelength
→ only certain wavelengths appear, as separate linesPart (b) — identifying elements
Each element has a unique line pattern (a fingerprint)
Matching the star’s spectral lines to known patterns…
…tells you which elements the star contains
💡 Top tips
Emission = bright on black; absorption = dark on colour. State both the line colour and the background — that’s where marks are won or lost
The lines match up. For one element, emission and absorption lines sit at the same wavelengths (inverse spectra)
Discrete lines are the evidence for discrete (quantised) energy levels — say “discrete/quantised” explicitly
Unique pattern = fingerprint — this is how we identify elements in stars and in the lab
⚠ Common mistakes
Describing absorption as bright lines — it’s dark lines on a bright, continuous background
Saying the spectrum is continuous — atomic spectra are discrete lines
Mixing up how each forms: heating/exciting a gas gives emission; white light through a cool gas gives absorption
Forgetting that emission and absorption lines coincide for the same element
Up next: we put numbers to those lines. The next page, Photon Energy, links each spectral line to an exact electron transition using E = hf and E = hc/λ — so you can calculate the wavelength a jump produces, and read an energy-level diagram like a map.
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