Emission, Absorption and Continuous Spectra: HSC Physics Guide
Learn how atomic transitions and thermal sources create emission, absorption, and continuous spectra. Understand how spectra reveal information about stars.
Imagine you are looking at two stars through a telescope. They both look like tiny points of white light, but one has a different chemical signature from the other. How can a thin line of colour tell you what a star is made of when the star is impossibly far away?
A first guess might be that a hotter object simply produces a different rainbow of colours. That explains some observations, but not the sharp lines astronomers actually measure. The missing idea is that atoms only interact with certain specific energies of light.
Understanding emission, absorption, and continuous spectra gives you a way to connect tiny atomic changes to large-scale observations, including how astronomers classify stars using stellar spectra.
01Light is not always a smooth rainbow
Start with a simple observation. If you pass sunlight through a prism, you see a continuous spread of colours from red through to violet.
A continuous spectrum contains all wavelengths in a range. A hot, dense object such as a glowing metal filament or the interior of a star can produce this type of spectrum because the material contains many atoms interacting closely together.
Now predict this:
If every star is made of atoms, and every atom can emit light, should every star produce a continuous rainbow?
The answer is no. The atoms in a thin gas behave differently from atoms packed closely in a hot solid or dense plasma. They can only emit and absorb certain energies of light.
02Building the atomic model
Picture an atom as having a staircase inside it.
An electron cannot stand halfway between steps. It can only occupy certain energy levels.
This is a simplified picture, but it is useful. The exact behaviour of electrons is described by quantum mechanics, but the staircase model helps explain spectra.
Suppose an electron starts on a low energy step and receives exactly the right amount of energy from incoming light. It can jump upward.
The atom has absorbed a photon.
The energy of that photon must match the energy difference between the two levels:
\[ E_{\text{photon}} = E_{\text{higher}} - E_{\text{lower}} \]
where:
- \(E_{\text{photon}}\) is the energy of the absorbed or emitted light,
- \(E_{\text{higher}}\) is the electron’s final energy level,
- \(E_{\text{lower}}\) is the electron’s starting energy level.
Because atoms have specific energy levels, they only absorb specific photon energies.
The reverse can also happen. If an electron falls from a higher energy level to a lower one, it releases a photon.
\[ E_{\text{photon}} = hf \]
where:
- \(h\) is Planck’s constant,
- \(f\) is the frequency of the emitted light.
Because frequency and wavelength are linked:
\[ c = f\lambda \]
where:
- \(c\) is the speed of light,
- \(\lambda\) is wavelength,
each electron transition produces a very specific wavelength of light.
That is why spectral lines are like atomic fingerprints.
03Emission spectra: atoms releasing light
Imagine a tube containing a low-pressure gas, such as hydrogen. When electricity passes through the gas, atoms gain energy. Electrons jump to higher energy levels.
But those excited electrons do not stay there forever. They fall back down and release photons.
The result is an emission spectrum.
An emission spectrum appears as bright lines on a dark background because only certain wavelengths are produced.
A student might predict that hydrogen should produce every colour because hydrogen atoms contain many electrons moving around. The problem is that hydrogen does not have unlimited possible energy changes. Its allowed energy levels restrict the possible photon energies.
04Absorption spectra: atoms removing light
Now imagine a hot, dense source producing a continuous spectrum. That light travels through a cooler, thin gas before reaching your detector.
The cooler gas contains atoms that can absorb certain wavelengths.
Those photons disappear from the continuous spectrum because the atoms use that energy to move electrons to higher levels.
The result is an absorption spectrum.
It looks like a continuous rainbow with dark lines missing.
The key difference is the direction of the energy transfer:
| Spectrum | What happens to photons? | Appearance |
|---|---|---|
| Continuous spectrum | Many wavelengths are produced | Smooth rainbow |
| Emission spectrum | Atoms release specific wavelengths | Bright lines |
| Absorption spectrum | Atoms remove specific wavelengths | Dark lines |
05Thermal sources and continuous spectra
A continuous spectrum does not come from atomic transitions between isolated energy levels in the same way that line spectra do.
Instead, it is produced by thermal radiation from a hot, dense object.
All objects emit thermal radiation because their particles are moving. The hotter the object, the more energy it emits, and the peak wavelength shifts.
For example:
- a cooler object may emit mostly infrared radiation,
- a very hot object can emit visible light,
- an extremely hot source can appear blue-white.
A useful model is a glowing metal bar. As it gets hotter, it does not suddenly switch from producing no light to producing one colour. It gradually emits more radiation across a range of wavelengths.
The model is still simplified. Real thermal spectra depend on the object’s temperature and properties, but the key HSC idea is that hot dense objects produce continuous spectra.
06How stars create absorption spectra
Stars combine both ideas.
The inside of a star is extremely hot and dense, so it produces a continuous spectrum.
That light then travels through the star’s cooler outer atmosphere. Atoms in this atmosphere absorb particular wavelengths.
When astronomers observe the star’s light, they see an absorption spectrum.
The missing wavelengths reveal which elements are present in the star’s atmosphere.
This is why spectra can reveal composition without collecting a sample from the star. The pattern of absorption lines acts like a chemical signature.
You can read more about how these spectral patterns are used to classify stars in the tutorgum guide on classifying stars with stellar spectra.
07A common misconception: “black lines mean the light is missing forever”
A dark absorption line does not mean the star stopped producing that wavelength.
The star’s interior still produces those photons.
The atoms in the cooler atmosphere absorb them, then may re-emit them in random directions. From our viewpoint, fewer of those photons arrive directly from the star.
The missing line is about what reaches the detector, not what the star creates.
08Worked example: finding the energy of an emitted photon
A hydrogen atom emits a photon with a frequency of \(6.17 \times 10^{14}\ \text{Hz}\). Calculate the energy of the photon.
Step 1
\[ E = hf \]
where \(E\) is photon energy, \(h\) is Planck’s constant \((6.63 \times 10^{-34}\ \text{J s})\), and \(f\) is frequency.
Step 2
\[ E = (6.63 \times 10^{-34})(6.17 \times 10^{14}) \]
Step 3
\[ E = 4.09 \times 10^{-19}\ \text{J} \]
Step 4
The atom released a photon with energy \(4.09 \times 10^{-19}\ \text{J}\). This energy corresponds to one specific electron transition, which produces one specific spectral line.
09Worked example: identifying an unknown gas
A laboratory gas produces bright emission lines at wavelengths of 410 nm, 434 nm, 486 nm, and 656 nm. A student compares this pattern with known spectra and finds the same four lines in hydrogen.
Identify the gas and explain why the match occurs.
Step 1
An emission spectrum depends on the allowed energy differences between electron energy levels.
Step 2
A single wavelength could accidentally match different elements. A complete pattern of lines is much stronger evidence.
Step 3
The gas is hydrogen because its electron transitions produce exactly these characteristic wavelengths.
Step 4
Hydrogen atoms have a unique set of energy levels. When electrons move between these levels, they emit photons with specific energies, creating a unique emission spectrum.
10Using spectra to investigate the universe
Spectra are not only about identifying elements. They can reveal information about stars and galaxies.
A star’s absorption lines can indicate:
- which elements are present,
- the star’s temperature,
- motion relative to Earth through shifts in wavelength.
This connects naturally to the way astronomers study stellar properties and evolution using tools such as the Hertzsprung-Russell diagram.
The central chain of reasoning is:
hot, dense source → continuous spectrum
cooler gas in front → absorption lines
excited atoms alone → emission lines
If you can follow the movement of energy, you can predict the spectrum.
11Questions and solutions
Question 1
A student observes a bright red line in the emission spectrum of a gas sample.
Explain why this line appears rather than a smooth range of red colours.
Solution 1
The red line appears because the gas atoms can only emit specific photon energies. Electrons in the atoms fall between allowed energy levels, releasing photons with particular energies.
A smooth range of red colours would require many different photon energies. Instead, the electron energy levels restrict the possible transitions, so only certain wavelengths are emitted.
Question 2
A hot object produces a continuous spectrum. Light from this object passes through a cooler hydrogen gas before reaching a detector.
Explain why the detector records dark lines in the spectrum.
Solution 2
The detector records dark lines because the hydrogen gas absorbs specific wavelengths from the continuous spectrum.
The tempting explanation is that the cooler gas blocks all light, but that would remove a broad range of wavelengths. The actual constraint is that hydrogen atoms only absorb photons whose energies match allowed electron transitions. Those missing wavelengths appear as dark absorption lines.
Question 3
Two stars have the same continuous spectrum source temperature, but only one star shows strong hydrogen absorption lines.
Which star likely has more hydrogen in its outer atmosphere? Explain your reasoning.
Solution 3
The star with stronger hydrogen absorption lines likely has more hydrogen in its outer atmosphere, assuming the observations are made under similar conditions.
The absorption lines form when photons from the hot interior pass through cooler gas. More hydrogen atoms available to absorb the correct photon energies generally produces stronger absorption features.
However, the line strength also depends on temperature because atoms must be in the correct energy state to absorb. The conclusion requires assuming the stars have similar atmospheric conditions apart from hydrogen abundance.
Question 4
A scientist observes a gas cloud producing bright emission lines. The scientist then heats the gas until it becomes much denser.
Predict how the observed spectrum changes and explain why.
Solution 4
The spectrum changes from a line emission spectrum towards a continuous spectrum because the denser, hotter material behaves more like a thermal source.
The tempting prediction is that the emission lines should simply become brighter. That would be true if the gas remained thin and isolated. The important change is density: when particles are packed closely together, many interactions occur, and the radiation becomes spread across a wider range of wavelengths rather than only individual atomic transitions.
Question 5
A student argues:
“Because every element has a unique emission spectrum, a star’s spectrum must always be an emission spectrum showing all the elements inside it.”
Evaluate this claim.
Solution 5
The claim is incorrect because a star’s observed spectrum is usually an absorption spectrum, not an emission spectrum, and it reveals mainly the elements in the star’s cooler outer atmosphere.
The student’s reasoning starts with a correct idea: atoms do have unique spectral fingerprints. The mistake is assuming that all atoms in the star are producing visible emission lines.
A star contains a very hot, dense interior that produces a continuous spectrum. As this light passes through the cooler outer layers, atoms there absorb specific wavelengths, creating dark lines.
The deeper constraint is the location and condition of the atoms. The same element can produce an emission spectrum in a thin excited gas, but produce absorption lines when cooler gas sits in front of a continuous source.
So the spectrum does not simply show every element everywhere inside the star. It shows which wavelengths are removed or added under the conditions where the light interacts with matter.
The next step is using these spectral clues to determine properties of stars, including their temperature, composition, and motion through space.