25.3.1Medical physics and astronomy (A Level)
Emission Spectra
5 min read 10 parts Lesson 18 of 22 1 quiz
Lesson summary
25.3.1 Emission Spectra — AS PHYSICS 9702 notes: Describe emission and absorption line spectra, explain how they arise from energy levels and how they are used in astronomy.Continuous spectrum — It contains every wavelength in a range and comes from a hot dense body such as a fil…
- Line emission spectrum
- Line absorption spectrum
- Fingerprint
- Discrete levels
- Negative values
- Ground and excited states
Section 1 of 10Three kinds of spectrum
Three kinds of spectrum
Continuous spectrum
- It contains every wavelength in a range and comes from a hot dense body such as a filament lamp or a star's surface.
Line emission spectrum
- Bright narrow lines on a dark background from a hot, low-pressure gas of excited atoms.
Line absorption spectrum
- A continuous spectrum crossed by dark lines when light from a hot source passes through a cooler gas.
Fingerprint
- Each element produces its own pattern, so a spectrum tells astronomers what a distant star is made of.
Section 2 of 10Energy levels in atoms
Discrete levels
- Electrons in an isolated atom have only certain energies.
Negative values
- The zero of energy is a free electron at rest just outside the atom.
Ground and excited states
- The lowest level is the ground state and higher levels are excited states.
Hydrogen
- The levels are −13.6 eV, −3.40 eV, −1.51 eV, −0.85 eV and so on, getting closer together towards zero.
Transitions
- An electron moves between levels only by gaining or losing exactly the energy difference.
Section 3 of 10Emission of photons
1Process
- An excited electron falls from E2 to E1 and the atom emits a single photon equal to the gap.
2Photon energy
- Photon energy hf = E2 − E1, so λ = hc/(E2 − E1).
3Units
- Energies must be in joules (1 eV = 1.60 × 10−19 J) and λ in metres.
4Gap and photon
- A large gap gives a high-frequency, short-wavelength photon.
Section 4 of 10Absorption of photons
1Process
- An atom absorbs a photon only if its energy exactly equals the gap between two levels.
2Dark lines
- Those photons are removed from the beam, leaving dark lines at the wavelengths where the gas would emit bright lines.
3Re-emission
- The atoms re-emit photons in all directions, so the intensity in the original direction is reduced.
4Same wavelengths
- The wavelengths missing from an absorption spectrum are the same as the emission lines of the same element.
Section 5 of 10Why there are lines
Discrete energies
- Only a limited set of photon energies is possible, so only a limited set of wavelengths appears.
Evidence
- Lines are direct evidence that atomic energy levels are quantised.
Number of transitions
- With N levels there can be up to N(N − 1)/2 transitions, though not all are equally bright.
Brightness
- It depends on how many atoms make that transition.
Section 6 of 10Spectra in astronomy
Starlight
- A star's atmosphere is cooler than the layers below, so its spectrum is a continuous curve crossed by absorption lines.
Elements
- Elements in the star, mainly hydrogen and helium, absorb some of the emitted wavelengths.
Identification
- The lines identify the elements present, and their strengths give information about temperature.
Distant galaxies
- Their lines show the same pattern as the Sun's but shifted to longer wavelengths.
Redshift
- Systematic shifts compared with laboratory wavelengths reveal the motion of the galaxy.
Section 7 of 10Describing and using a spectrum
1
Identify the type of spectrum, with bright lines for emission and dark lines on a bright band for absorption.
2
Convert energies to joules before using hc/λ and keep the wavelength in metres.
3
Check the sign, with the lower level subtracted from the upper level for emission.
4
Check the wavelength is sensible, since visible light is about 400 nm to 700 nm and a 2 eV gap is visible while 10 eV is ultraviolet.
Section 8 of 10More from the specification
1
Stellar spectra evolution over years
- Stellar spectra evolution over years analyzing stellar atomic emission and absorption lines traces elemental composition changes spanning millions of evolutionary years.
2
Redshift and the Doppler Effect in Astronomy
- Light from distant galaxies exhibits Doppler redshift, indicating that celestial sources are moving radially away from observers on Earth.
- (Δλ / λ₀ ≈ v / c)
3
Hubble's Law and Galactic Recession
- Hubble's law states that the recession speed v of a distant galaxy is directly proportional to its distance d from Earth (v = H₀·d),
- where H₀ is Hubble's constant.
4
Expansion of Space and Big Bang Theory
- Cosmological redshift occurs because the fabric of space itself is stretching during cosmological expansion;
- extrapolated backwards, this provides primary observational evidence supporting the Big Bang theory of the universe.
5
Doppler effect
- Change in observed wavelength and frequency due to the motion of a source.
6
Expanding Universe
- Space is stretching so that galaxies move apart.
7
Universe
- All of space and everything in it.
8
Theory
- A well-tested, wide-ranging explanation supported by evidence.
9
Expansion
- Increase in size on heating as particles move further apart.
10
Recession
- Movement away from the observer.
11
Constant
- Not changing.
12
v = H0d
What the symbols mean: v = recession speed of the galaxy (km s−1) · H0 = Hubble constant (km s−1 Mpc−1) · d = distance (Mpc)
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Continuous spectrum | A spectrum containing every wavelength in a range. |
| Line emission spectrum | A set of bright narrow lines on a dark background from an excited gas. |
| Line absorption spectrum | A continuous spectrum crossed by dark lines at wavelengths absorbed by a cooler gas. |
| Energy level | One of the discrete energies an electron in an atom can have. |
| Photon | A quantum of electromagnetic radiation with energy E = hf. |
Section 10 of 10Exam technique
1
Using energies in eV in λ = hc/ΔE without converting to joules — convert 1 eV = 1.60 × 10−19 J first.
2
Treating energy levels as positive and getting a negative difference — subtract the lower level from the higher.
3
Saying dark absorption lines are at different wavelengths from bright lines of the same element — they are at the same wavelengths.
4
Thinking photons of any energy above the gap can be absorbed — only the exact energy is absorbed in a transition between bound levels.
5
Confusing a continuous spectrum with a line spectrum — dense hot sources give continuous spectra and low-pressure gases give lines.