Lesson summary
22.4.2 Line Spectra — AS PHYSICS 9702 notes: Describe continuous, emission and absorption line spectra and explain how each line arises from a transition between energy levels.Continuous spectrum — All wavelengths in a range, produced by hot solids such as a filament lamp.Atoms i…
- Line emission spectrum
- Line absorption spectrum
- One transition, one line
- Brightness
- Re-emission
- Dark lines
Section 1 of 9Three kinds of spectrum
Three kinds of spectrum
Continuous spectrum
- All wavelengths in a range, produced by hot solids such as a filament lamp.
- Atoms in a solid interact strongly, so energy levels merge into broad bands.
Line emission spectrum
- Coloured lines at specific wavelengths on a dark background, from a hot, low-pressure gas of isolated atoms.
Line absorption spectrum
- A continuous background crossed by dark lines at specific wavelengths, seen when white light passes through a cooler gas.
Line spectra
- Each element produces its own unique set of lines, so an element can be identified by its spectrum.
Section 2 of 9Emission spectra
1Process
- Collisions excite atoms in a discharge tube, and each atom de-excites by emitting photons.
2One transition, one line
- A transition from E2 to E1 gives a photon with hf = E2 − E1, so λ = hc/(E2 − E1).
3Brightness
- A line is bright when many atoms make that transition per second.
- Brightness depends on how easily the upper level is populated and how likely the transition is.
4Evidence
- The discrete wavelengths show that electrons transition only between discrete energy levels.
Section 3 of 9Absorption spectra
1Process
- White light passes through a cool, low-pressure gas, and photons matching an energy gap are absorbed.
2Re-emission
- The excited atoms re-emit photons in all directions, not in the original direction.
- So some wavelengths appear to be missing.
3Dark lines
- They appear at the same wavelengths as the bright lines in the emission spectrum of the same element.
4Fraunhofer lines
- Cooler outer layers of the Sun absorb specific wavelengths from the hot interior.
- Matching these to laboratory spectra identifies the elements in stars.
Section 4 of 9Using a diffraction grating
Purpose
- A grating separates the wavelengths of light from a gas.
Grating equation
- Grating equation d sin θ = nλ, where d is the grating spacing, θ the angle to the n-th order and n the order number.
Finding the gap
- Measuring θ gives λ and hence the energy gap.
Grating spacing
- Grating spacing d is the reciprocal of the number of lines per metre.
Section 5 of 9Reading a spectrum
Axes
- Spectra are drawn as vertical lines along a wavelength or frequency axis.
Energy and wavelength
- Short-wavelength lines come from large energy gaps.
Hydrogen
- Transitions down to n = 1 give ultraviolet lines and transitions down to n = 2 give the visible Balmer series.
Number of lines
- It is limited to the transitions that occur, and it can help estimate the number of levels.
Line width
- Lines are treated as single wavelengths at this level.
Section 6 of 9Applications
Identifying elements
- Every element has its own set of lines, used in stars, forensic samples and chemical analysis.
Reference catalogue
- Comparing an unknown spectrum with a catalogue is the standard identification method.
Redshift
- A shift of known line patterns to longer wavelengths shows the recession of galaxies.
Section 7 of 9More from the specification
Quantized Atomic Energy Levels
- In an isolated atom, electrons occupy discrete energy states called atomic energy levels, represented on an energy level diagram as horizontal lines with specific negative values.
Photon Emission and Energy Difference
- When an electron undergoes a downward transition between certain energy levels, a single photon is emitted whose energy corresponds exactly to the energy difference ΔE = Einitial - Efinal.
Line Spectra and Transition Wavelengths
- Calculating the emitted photon's frequency and wavelength using ΔE = h·f = h·c / λ reveals that discrete spectral lines correspond to unique transitions,
- giving each element an identifiable optical fingerprint.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Continuous spectrum | A spectrum containing all wavelengths in a range, from a hot solid. |
| Line emission spectrum | Bright lines at specific wavelengths on a dark background, from excited gas atoms. |
| Line absorption spectrum | A continuous spectrum crossed by dark lines at the wavelengths absorbed by a cool gas. |
| Spectral line | A single wavelength produced or absorbed by one transition between two energy levels. |
| Diffraction grating | A surface with many equally spaced slits that separates light into its wavelengths. |
Section 9 of 9Exam technique
1
Saying dark absorption lines mean the light was destroyed — it was absorbed and re-emitted in random directions.
2
Describing a hot solid as giving a line spectrum — dense hot solids give continuous spectra and low-pressure gases give lines.
3
Forgetting that each line is one specific energy gap — it is not a range of energies.
4
Using the grating spacing as lines per metre — d is the reciprocal of the lines per metre.
5
Thinking shorter-wavelength lines come from smaller energy gaps — they come from larger gaps.