Key term
WJEC AS-Level Physics 1420QS
Basic Physics
Kinematics
Dynamics
Energy Concepts
Solids Under Stress
Using Radiation to Investigate Stars
Particles and Nuclear Structure
Topic overviewQuarks, leptons and hadronsThe four interactions and conservation lawsThe first generation of quarks and leptonsAntiparticles and annihilationBaryons, antibaryons and mesonsDeducing quark compositions from chargeBeta decay and quark flavour changeTesting a reaction against the conservation rules
Conduction of Electricity
Resistance
Topic overviewCalculating resistance from resistivityPotential difference, Ohm's law and powerResistivity, temperature and superconductivityThe volt: energy transferred per unit chargeCurrent–voltage characteristics of non-ohmic componentsMeasuring resistance and resistivity in the laboratoryThe heating effect of a current and choosing a power equationThermistors and how a semiconductor's resistance falls with temperatureConductance, conductivity and comparing materials
D.C. Circuits
Topic overviewKirchhoff's laws, and resistors in series and parallelPotential dividers, emf and internal resistanceCells, resistivity, power and energyWorking out currents and potential differences in resistor networksSensor circuits with thermistors and LDRsInternal resistance, power transfer and short-circuit currentAmmeters, voltmeters and their effect on the circuit
The Nature of Waves
Wave Properties
Topic overviewEnergy transfer and the wave equationTransverse and longitudinal waves, path difference and intensityDiffraction and superpositionTwo-source interference and the diffraction gratingStationary wavesHow much a wave spreads at a gap or an obstacleCoherence and the conditions for observable interferenceYoung's double slit and λ = ay/DMeasuring wavelength with a diffraction gratingMeasuring the speed of sound with stationary waves
Refraction of Light
Photons
Topic overviewPhoton energy and the electronvoltThe photoelectric effectStopping potential and the Planck constantThe photoelectric effect and photon energiesLine spectra and energy levelsDemonstrating the photoelectric effectThe electromagnetic spectrum and typical photon energiesElectron diffraction and the de Broglie relationshipRadiation pressure
Lasers
Mathematical Requirements (Appendix C)
Topic overviewSignificant figures and arithmetic meansUncertainty and how uncertainties combineAlgebraic symbols and proportionalityRearranging and solving equationsSmall angle approximationsTangents and instantaneous rates of changeAreas under curves and numerical methodsReading a tangent on a motion graphStraight-line graphs and y = mx + cWhat a gradient and an intercept representLinearising a relationship to find a constantLog plots of exponential decayChoosing what to plot to find a constantLogarithmic scales and orders of magnitudeWorking with base-ten logarithmsThe shapes of common graphsInverse and root relationships on a graphAreas, volumes and PythagorasGeometry applied to physical situationsSubstituting into physical equationsTrigonometry and right-angled trianglesResolving a force into componentsRadians, arc length and force diagramsRandom processes and translating between representationsMean values and periodic functionsSymbols for approximation and comparison
Lasers
- 5 subtopics
- 15 things to know
- AS Unit 2
What Lasers covers
Lasers is examined on AS Unit 2 of WJEC Physics, Electricity and Light. 61 recall questions across 5 subtopics.
The 5 subtopics
Each one has its own page. WJEC AS Physics students sit this topic in the first year.
- Stimulated emission and the properties of laser lightRecall questions on Stimulated emission and the properties of laser light.Open →
- Population inversion, pumping and the laser cavityRecall questions on Population inversion, pumping laser cavity and the laser cavity.Open →
- Why a two-level system cannot give a population inversionRecall questions on Why a two-level system cannot give a population inversion.Open →
- Three-level and four-level laser systemsRecall questions on Three-level and four-level laser systems.Open →
- The semiconductor laser and its usesRecall questions on The semiconductor laser and its uses.Open →
What you need to know
Answer each one before you reveal it.
Your recallNot started
Process
How is a laser cavity built, and what happens to the light inside it?
Key term
Why does adding a third energy level make a population inversion possible?
Key term
In a real gas laser, why is the population inversion described as being between two particular levels rather than across the whole atom?
Key term
What is a metastable state?
Key term
Why must more than half of all the atoms be lifted out of the ground state before a three-level laser can work?
Key term
What forms the two mirrors of the cavity in a semiconductor laser?
Key term
Why does the high efficiency of a semiconductor laser matter in a portable device?
Key term
Describe how a laser reads the data on an optical disc.
Process
What fundamental process does a laser use to produce light?
Key term
What is population inversion?
Key term
In stimulated emission, what must be present to interact with an excited atom?
Key term
What would happen to laser operation if population inversion were not achieved?
Key term
In a collection of atoms at thermal equilibrium, which of two energy levels holds more atoms?
Key term
Name a laser that uses a three-level system.
Take Lasers further
The same content, four other ways.
Commonly confused
Pairs that lose marks when they are blurred together.
Spontaneous emission vs Stimulated emission
Spontaneous emission: the random decay of an excited atom, producing light of no fixed phase. Stimulated emission: decay triggered by a passing photon, producing light of the same phase and direction.
Questions from this topic
Each one with the model answer underneath.
- 1
What fundamental process does a laser use to produce light?
Model answer
Stimulated emission of radiation.
- 2
In stimulated emission, what must be present to interact with an excited atom?
Model answer
An incoming photon.
- 3
During stimulated emission, what happens to the excited atom after interacting with the incoming photon?
Model answer
The atom drops to a lower energy level.
- 4
How does the frequency of the emitted photon in stimulated emission compare to that of the incident photon?
Model answer
The emitted photon has the same frequency as the incident photon.
- 5
What is the phase relationship between the emitted photon and the incident photon in stimulated emission?
Model answer
The emitted photon has the same phase as the incident photon.
- 6
In what direction does the emitted photon travel relative to the incident photon during stimulated emission?
Model answer
The emitted photon travels in the same direction as the incident photon.
What is inside Levo
Six ways to work the same material.
Lasers revision questions
- Is Lasers examined at AS in WJEC Physics?
- Yes. It sits in AS Unit 2, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
- What does Lasers cover in WJEC AS-Level Physics?
- 5 subtopics: stimulated emission and the properties of laser light, population inversion, pumping and the laser cavity, why a two-level system cannot give a population inversion, three-level and four-level laser systems and the semiconductor laser and its uses. Each one has its own page with questions and model answers.
- How is Lasers organised?
- By the WJEC 1420QS specification, subtopic by subtopic, so it matches the order you are taught it in.
Every question in Lasers, on a schedule.
Attempt a question from memory, write the correct version, then attempt it again. Levo records the jump between your first attempt and your last, and brings each question back on the day you are due to forget it.
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