AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Line spectra (eg of atomic hydrogen) as evidence for transitions between…
Full specification wording
Energy levels and photon emission: Line spectra (eg of atomic hydrogen) as evidence for transitions between discrete energy levels in atoms.
A line spectrum is a series of sharp, separate bright lines on a dark background, each at a precise wavelength.
Checked against the AQA 7408 specification (opens in a new tab) on .
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A line spectrum is a series of sharp, separate bright lines on a dark background, each at a precise wavelength. For atomic hydrogen, the visible Balmer lines and the ultraviolet Lyman lines are measured and tabulated. A continuous spectrum would be expected if an electron could have any energy, but sharp lines show that only certain energy changes occur. Each line arises when an electron drops from a higher discrete level to a lower one, emitting one photon of energy hf equal to the level difference. Because the levels are discrete, only certain photon energies, and hence wavelengths, appear. The spectrum therefore acts as evidence for discrete energy levels and for transitions between them.
What earns the marks
- A line spectrum consists of sharp, separate lines at precise wavelengths, not a continuous spread of wavelengths.
- Each observed line corresponds to a single photon emitted when an electron makes one transition between two discrete energy levels.
- The photon energy is given by hf = E_initial − E_final, so the wavelength is fixed by the energy gap.
- Because only certain energy gaps exist, only certain photon energies and wavelengths are emitted, producing discrete lines.
- The pattern of lines, for example the Balmer series in hydrogen, can be used to deduce the energy level spacings.
- A continuous spectrum would imply a continuous range of possible electron energies, which is not observed.
Common mistakes and how to put them right
Treating the lines as showing that electrons have continuous energies.
Put it right: Sharp lines show discrete energy levels.
Saying a line is produced when an electron absorbs energy and moves up.
Put it right: An emission line is produced when an electron moves down and emits a photon.
Confusing the energy of a level with the energy of a transition.
Put it right: The photon energy equals the difference between two levels, not the energy of one level.
Exam tips
- State clearly that the lines are evidence for discrete energy levels, then link each line to one transition.
- Use hf = E_1 − E_2 and c = fλ when asked to calculate a wavelength or energy gap.
- Describe the spectrum as sharp lines on a dark background, and contrast this with a continuous spectrum.
Try the lesson
The start of the Energy levels and photon emission: Line spectra (eg of atomic hydrogen) as evidence for transitions between discrete energy levels in atoms. lesson, made from checked teaching notes for this specification point.
Have a go first
A line spectrum is observed from a sample of atomic hydrogen. What does the presence of separate sharp lines, rather than a continuous band of colours, show about the energy levels in a hydrogen atom?
Listen
A line spectrum is a series of sharp, separate bright lines on a dark background, each at a precise wavelength. For atomic hydrogen, the visible Balmer lines and the ultraviolet Lyman lines are measured and tabulated. A continuous spectrum would be expected if an electron could have any energy, but sharp lines show that only certain energy changes occur. Each line arises when an electron drops from a higher discrete level to a lower one, emitting one photon of energy hf equal to the level difference. Because the levels are discrete, only certain photon energies, and hence wavelengths, appear. The spectrum therefore acts as evidence for discrete energy levels and for transitions between them. A strong answer earns marks for these points. A line spectrum consists of sharp, separate lines at precise wavelengths, not a continuous spread of wavelengths. Each observed line corresponds to a single photon emitted when an electron makes one transition between two discrete energy levels. The photon energy is given by hf = E_initial − E_final, so the wavelength is fixed by the energy gap. Because only certain energy gaps exist, only certain photon energies and wavelengths are emitted, producing discrete lines. A common slip: Treating the lines as showing that electrons have continuous energies. Instead, sharp lines show discrete energy levels. Exam tip: State clearly that the lines are evidence for discrete energy levels, then link each line to one transition.
Key words
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Line spectrum
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Discrete energy levels
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Photon emission
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Spectroscopic evidence
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See every definition
- Line spectrum
- A series of sharp, separate bright lines on a dark background, each at a precise wavelength.
- Discrete energy levels
- Only certain energy changes occur, shown by sharp lines rather than the continuous spread a continuous spectrum would imply.
- Photon emission
- Each line arises when an electron drops from a higher discrete level to a lower one, emitting one photon of energy hf equal to the level difference.
- Spectroscopic evidence
- The pattern of lines, such as the Balmer series in hydrogen, can be used to deduce the energy level spacings.
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