Numbers
CCEA A-Level Physics 1210
Physical Quantities, Scalars and Vectors
Moments and Equilibrium
Linear Motion and Dynamics
Newton's Laws of Motion
Linear Momentum and Impulse
Work, Energy and Power
Current, Charge and Potential Difference
Resistance and Resistivity
Topic overviewResistance and Ohm's lawOhmic and non-ohmic behaviourResistors in series and in parallelInvestigating current, voltage and resistance in series and parallel circuitsElectrical power and energy in resistive circuitsResistivity and the equation R = ρl/AMeasuring the resistivity of a metalCurrent–voltage characteristics of a conductor, a lamp and a diodeResistance, temperature and superconductivity
Internal Resistance and Potential Dividers
Topic overviewInternal resistance and terminal potential differenceConsequences of internal resistance for external circuitsMeasuring electromotive force and internal resistanceThe potential divider and variable outputPotential dividers in lighting and heating control circuitsLoaded potential dividers and output voltage calculations
Waves
Refraction
Lenses
Superposition, Interference and Diffraction
Topic overviewSuperposition of wavesStanding waves, nodes and antinodesStanding waves on stretched stringsStanding waves in a pipe closed at one endMeasuring the speed of sound with a resonance tubeConditions for observable interferencePath difference and phase differenceYoung's double slitDiffraction at a single slitThe diffraction grating
Quantum Physics
Wave-Particle Duality
Astronomy
Practical Skills and Data Analysis
Topic overviewChoosing and using measuring instrumentsPrecision, accuracy and recording measurementsUsing the cathode ray oscilloscope to measure voltage and frequencyTables, graphs and presenting resultsLinearising a relationship and using the gradientAbsolute and percentage uncertaintyCombining uncertainties and uncertainty on a graphRandom and systematic error, reliability and refining an experiment
Deformation of Solids
Thermal Physics
Topic overviewBoyle's law, Charles's law and the pressure lawThe ideal gas equation pV = nRTThe Avogadro constant, the mole and pV = NkTInternal energy and the distribution of molecular energyThe kinetic theory equation pV = ⅓Nm⟨c²⟩Average molecular kinetic energy and temperatureAbsolute zero and the kelvin scaleSpecific heat capacity and the electrical method
Uniform Circular Motion
Simple Harmonic Motion
Topic overviewDefining simple harmonic motion and a = −ω²xDisplacement, velocity and acceleration: x = A cos ωtThe simple pendulum and T = 2π√(l/g)The loaded spiral spring and T = 2π√(m/k)SHM graphs and finding velocity from a gradientFree vibrations, forced vibrations and resonanceDamping: light, critical and over-damping
The Nucleus and Radioactive Decay
Topic overviewAlpha-particle scattering and the nuclear atomNuclear radius and nucleon numberThe density of nuclear matterAlpha, beta and gamma radiation: nature, penetration and rangeNuclear equations and changes in nucleon and proton numberThe random and exponential nature of radioactive decayActivity, the decay constant and A = −λNExponential decay calculations with A = A₀e^(−λt)Half-life and measuring it
Nuclear Energy, Fission and Fusion
Gravitational Fields
Electric Fields
Capacitors
Magnetic Fields
Charged Particles and Accelerators
Lenses
The power of a lens
- 3 things to know
- Unit AS 2
Overview
Recall questions on the definition of lens power and the equation P = 1/f with the dioptre, the signs for a converging and a diverging lens, worked conversions both ways, combining two thin lenses in contact, why opticians specify power rather than focal length, and what a negative prescription indicates.
What you need to know
Answer each one before you reveal it.
Your recallNot started
Key term
Why do opticians specify lenses by power rather than by focal length?
Numbers
Calculate the focal length of a lens of power +50 D and comment on the result.
Questions from this subtopic
Each one with the model answer underneath.
- 1
Define the power of a lens.
Model answer
The power of a lens is the reciprocal of its focal length, and it is a measure of how strongly the lens converges or diverges light.
- 2
State the equation for the power of a lens, defining each symbol and giving the unit.
Model answer
P = 1/f, where P is the power in dioptres (D) and f is the focal length in metres, so one dioptre is one m⁻¹.
- 3
What sign does the power of a converging lens have, and what sign does a diverging lens have?
Model answer
A converging lens has a positive power and a diverging lens a negative power, following the sign of the focal length.
- 4
Calculate the power of a converging lens of focal length 0.25 m.
Model answer
P = 1/f = 1/0.25 = +4.0 D, and 1/4.0 D returns a focal length of 0.25 m as given.
- 5
A lens has a power of −2.5 D. Calculate its focal length and state the type of lens.
Model answer
f = 1/P = 1/(−2.5) = −0.40 m, so it is a diverging lens of focal length 0.40 m, and 1/(−0.40 m) returns −2.5 D.
- 6
Which lens bends light more strongly, one of power +2.0 D or one of power +5.0 D?
Model answer
The +5.0 D lens, because a higher power means a shorter focal length, 0.20 m rather than 0.50 m, and therefore a greater deviation of the light.
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