Energy levels and photon emission — AQA A-Level Physics

This topic explores the quantised nature of energy in atoms. You will learn that electrons in an atom can only exist at specific, discrete energy levels, and that photons are emitted or absorbed when electrons transition between these levels. The energy of the photon is directly related to the energy difference between the levels, a concept fundamental to understanding atomic spectra and the behaviour of matter at the quantum scale.

Understanding energy levels and photon emission is crucial for explaining phenomena such as emission and absorption spectra, the operation of lasers, and the photoelectric effect. It forms the basis for more advanced topics in quantum physics and is a key part of the AQA A-Level Physics specification, often examined through calculations and explanations of spectra.

• →Electrons in an atom can only occupy certain discrete energy levels, meaning their energy is quantised.
• →An electron can transition between energy levels by absorbing or emitting a photon of energy exactly equal to the difference between the levels.
• →The energy of a photon is given by E = hf, where h is the Planck constant and f is the frequency of the photon.
• →The emission spectrum of an element consists of discrete lines corresponding to specific electron transitions from higher to lower energy levels.
• →The absorption spectrum consists of dark lines on a continuous background, corresponding to electrons absorbing photons and moving to higher energy levels.

• 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.

• 💡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.
• 💡Always write the equation hf = E1 - E2 when explaining photon emission, and explicitly state that the photon energy equals the energy difference between the two levels.
• 💡In calculations, convert all units to SI (joules, metres, hertz) before substituting into equations. Show your conversion steps clearly to avoid losing marks.
• 💡When describing spectra, use precise language: 'discrete lines' not 'lines that are separate', and 'specific frequencies' not 'certain colours'.

• Treating the lines as showing that electrons have continuous energies; the correction is that sharp lines show discrete energy levels.
• Saying a line is produced when an electron absorbs energy and moves up; the correction is that 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; the correction is that the photon energy equals the difference between two levels, not the energy of one level.
• Students often think electrons can have any energy value, but in fact they are restricted to specific discrete energy levels. Correction: Energy levels are quantised; only certain energy values are allowed.
• Students sometimes believe that a photon is emitted when an electron moves to a higher energy level. Correction: A photon is emitted when an electron moves to a lower energy level; absorption occurs when moving to a higher level.
• Students may think the energy of the emitted photon depends on the energy of the higher level alone, rather than the difference between levels. Correction: The photon energy equals the difference between the two energy levels involved in the transition.

1. 1Step 1: Review the basics of the photon model and the equation E = hf. Ensure you can convert between eV and J.
2. 2Step 2: Learn the energy level diagram for hydrogen and practice drawing transitions. Understand that energy levels are negative and that the energy difference is positive.
3. 3Step 3: Work through calculations involving energy differences, frequency, and wavelength. Practice converting units correctly.
4. 4Step 4: Study emission and absorption spectra, and be able to explain the origin of discrete lines in terms of electron transitions.
5. 5Step 5: Attempt past paper questions on this topic, focusing on both calculations and explanations. Review mark schemes to understand what examiners expect.

• 📋Calculation questions: These often ask you to calculate the energy, frequency, or wavelength of a photon emitted during a transition. Advice: Always convert units to SI and show your working clearly.
• 📋Explanation questions: These ask you to explain why emission spectra consist of discrete lines or to describe the process of photon emission. Advice: Use precise terminology and refer to energy levels and energy differences.
• 📋Diagram questions: You may be asked to draw energy levels and indicate transitions. Advice: Label levels with energies and show arrows for transitions, indicating direction (emission or absorption).
• 📋Comparison questions: These ask you to compare emission and absorption spectra. Advice: Mention that emission spectra are bright lines on a dark background, while absorption spectra are dark lines on a continuous spectrum, and relate both to electron transitions.