What is meant by the path difference between two waves?

The difference in distance travelled by two waves from their sources to a given point.

Define coherent sources.

Sources that have a constant phase difference and the same frequency.

Why must sources be coherent to produce a stable interference pattern?

If sources are not coherent, the phase difference between them varies randomly, causing the interference pattern to shift continuously and average out, so no stable pattern is observed.

What path difference condition produces constructive interference?

A path difference equal to a whole number of wavelengths (nλ), so the waves arrive in phase.

Explain why constructive interference occurs when the path difference is nλ.

When the path difference is a whole number of wavelengths, the waves arrive in phase, meaning their crests and troughs align, so they reinforce each other.

What path difference condition produces destructive interference?

A path difference equal to an odd number of half wavelengths ((n+½)λ), so the waves arrive in antiphase.

AQA A-Level Physics 7408

Measurements and Their Errors

Topic overview Use of SI units and their prefixes Limitation of physical measurements Estimation of physical quantities

Particles and Radiation

Topic overview Constituents of the atom Stable and unstable nuclei Particles, antiparticles and photons Particle interactions Classification of particles Quarks and antiquarks Applications of conservation laws The photoelectric effect Collisions of electrons with atoms Energy levels and photon emission Wave-particle duality

Waves

Topic overview Progressive waves Longitudinal and transverse waves Principle of superposition of waves and formation of stationary waves Interference Diffraction Refraction at a plane surface

Mechanics and Materials

Topic overview Scalars and vectors Moments Motion along a straight line Projectile motion Newton's laws of motion Momentum Work, energy and power Conservation of energy Bulk properties of solids The Young modulus

Electricity

Topic overview Basics of electricity Current–voltage characteristics Resistivity Circuits Potential divider Electromotive force and internal resistance

Further Mechanics and Thermal Physics

Topic overview Circular motion Simple harmonic motion (SHM)Simple harmonic systems Forced vibrations and resonance Thermal energy transfer Ideal gases Molecular kinetic theory model

Fields and Their Consequences

Topic overview Fields Newton's law Gravitational field strength Gravitational potential Orbits of planets and satellites Coulomb's law Electric field strength Electric potential Capacitance Parallel plate capacitor Energy stored by a capacitor Capacitor charge and discharge Magnetic flux density Moving charges in a magnetic field Magnetic flux and flux linkage Electromagnetic induction Alternating currents Transformers

Nuclear Physics

Topic overview Rutherford scattering α, β and γ radiation Radioactive decay Nuclear instability Nuclear radius Mass and energy Induced fission Safety aspects

AstrophysicsOptional topic

Topic overview Astronomical telescope consisting of two converging lenses Reflecting telescopes Single dish radio telescopes, I-R, U-V and X-ray telescopes Advantages of large diameter telescopes Classification by luminosity Absolute magnitude, M Classification by temperature, black-body radiation Principles of the use of stellar spectral classes The Hertzsprung-Russell (HR) diagram Supernovae, neutron stars and black holes Doppler effect Hubble's law Quasars Detection of exoplanets

Medical PhysicsOptional topic

Engineering PhysicsOptional topic

Topic overview Concept of moment of inertia Rotational kinetic energy Rotational motion Torque and angular acceleration Angular momentum Work and power First law of thermodynamics Non-flow processes The p–V diagram Engine cycles Second Law and engines Reversed heat engines

Turning Points in PhysicsOptional topic

Topic overview Cathode rays Thermionic emission of electrons Specific charge of the electron Principle of Millikan's determination of the electronic charge, e Newton's corpuscular theory of light Significance of Young's double slits experiment Electromagnetic waves The discovery of photoelectricity Wave–particle duality Electron microscopes The Michelson-Morley experiment Einstein's theory of special relativity Time dilation Length contraction Relativistic mass and energy

ElectronicsOptional topic

Topic overview MOSFET (metal-oxide semiconducting field-effect transistor)Zener diode Photodiode Hall effect sensor Difference between analogue and digital signals LC resonance filters The ideal operational amplifier Inverting amplifier configuration Non-inverting amplifier configuration Summing amplifier configuration Real operational amplifiers Combinational logic Sequential logic Astables Principles of communication systems Transmission media Time-division multiplexing Amplitude (AM) and frequency modulation (FM) techniques

Waves

Interference

7 things to know
Papers 1 and 3

Overview

Coherence and path difference, Young's double slit experiment, fringe spacing, and laser safety.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

What is the fringe spacing formula, and where do bright fringes fall?

Numbers

What do λ, D and s stand for, and how does wavelength change the pattern?

Key term

What safety precautions apply when using a laser?

Process

Why does white light produce coloured fringes, and which colour lies furthest out?

Numbers

Why is the central fringe white in white light, and what does wider slit separation do?

Key term

How can interference of sound waves be demonstrated in a laboratory?

Key term

According to quantum theory, what dual nature does light exhibit?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by the path difference between two waves?
Model answer
The difference in distance travelled by two waves from their sources to a given point.
2
Define coherent sources.
Model answer
Sources that have a constant phase difference and the same frequency.
3
Why must sources be coherent to produce a stable interference pattern?
Model answer
If sources are not coherent, the phase difference between them varies randomly, causing the interference pattern to shift continuously and average out, so no stable pattern is observed.
4
What path difference condition produces constructive interference?
Model answer
A path difference equal to a whole number of wavelengths (nλ), so the waves arrive in phase.
5
Explain why constructive interference occurs when the path difference is nλ.
Model answer
When the path difference is a whole number of wavelengths, the waves arrive in phase, meaning their crests and troughs align, so they reinforce each other.
6
What path difference condition produces destructive interference?
Model answer
A path difference equal to an odd number of half wavelengths ((n+½)λ), so the waves arrive in antiphase.

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