AQA A-Level Physics: Waves and Interference Key Concepts

Progressive wave

A progressive wave is a wave that transfers energy through a medium without transferring matter.

Wave equation

Wave speed v = frequency f × wavelength λ (v = fλ).

Displacement vs Amplitude

Displacement is the distance from the equilibrium at any point, while amplitude is the maximum displacement.

Phase difference

Phase difference is the difference in the phase angle between two points on a wave, measured in degrees or radians.

Transverse vs Longitudinal waves

Transverse waves have oscillations perpendicular to the direction of energy transfer, while longitudinal waves have oscillations parallel.

Example of transverse wave

Transverse: light wave.

Example of longitudinal wave

Longitudinal: sound wave.

Law of reflection

Angle of incidence = angle of reflection.

Snell's Law

n1 sinθ1 = n2 sinθ2.

Light in denser medium

It slows down and bends towards the normal.

Diffraction significance

When the gap size is approximately equal to the wavelength.

Principle of superposition

The resultant displacement at a point is the vector sum of displacements from individual waves.

Conditions for constructive interference

Waves must be in phase and have a path difference of nλ.

Conditions for destructive interference

Waves must be out of phase by π (or 180°) and have a path difference of (n + ½)λ.

Coherence

Two wave sources are coherent if they have a constant phase difference and the same frequency.

Stationary wave

A stationary wave is formed by the superposition of two progressive waves with the same frequency and amplitude moving in opposite directions.

Nodes and antinodes

Nodes are points of zero amplitude. Antinodes are points of maximum amplitude.

First harmonic

The lowest frequency standing wave with one antinode and two nodes; ½ wavelength fits in the length of the string.

Absolute refractive index

n = c / v, where c is the speed of light in vacuum and v is the speed in the medium.

Critical angle formula

sin(c) = n₂ / n₁.

Conditions for total internal reflection

Light must travel from a more to less dense medium, and the angle of incidence must exceed the critical angle.

Fringe spacing in Young's experiment

w = λD / s.

Bright fringe in Young's experiment

Constructive interference where waves arrive in phase.

Dark fringe in Young's experiment

Destructive interference where waves arrive out of phase by π radians.

Diffraction grating equation

nλ = d sinθ.

Effect of increasing slits in diffraction grating

Produces sharper and more widely spaced maxima in the interference pattern.

Wave speed in denser medium

Wave speed decreases in a denser medium due to reduced wavelength; frequency remains constant.

Wavefronts

Wavefronts are lines that represent the crest or trough of a wave, perpendicular to the direction of energy transfer.

Longitudinal waves and polarization

Because their oscillations occur in the same direction as energy transfer, not perpendicular like transverse waves.

Central diffraction maximum

The central diffraction maximum becomes wider and the intensity decreases.

Laser light in Young's experiment

Laser light is monochromatic and coherent, producing clear, stable interference fringes.

Effect of increasing slit separation

Fringe spacing decreases, as w = λD/s and s increases.

Change in refractive index

It depends on the optical density of the material and the wavelength of light.

Effect of increasing wavelength on fringe spacing

Fringe spacing increases since w = λD/s.

Multiple slits in diffraction gratings

To produce sharper and more intense interference fringes.

Monochromatic light

Light that consists of a single wavelength and frequency.

Safety considerations for lasers

Avoid direct eye exposure, use low-power lasers, and use beam stops or barriers.

Light bending towards normal

Because its speed decreases, and the change in direction is towards the normal due to refraction.

First harmonic in a closed tube

There is a node at the closed end and an antinode at the open end; it corresponds to ¼ wavelength.

Effect of damping on stationary wave

Damping reduces the amplitude of antinodes over time, eventually stopping the wave if not driven.

Wave

A repeating disturbance that transfers energy without transferring matter.

Transverse wave

A wave where the oscillations are perpendicular to the direction of energy transfer.

Longitudinal wave

A wave where the oscillations are parallel to the direction of energy transfer.

Displacement

Distance from the undisturbed position of a point on a wave.

Amplitude

Maximum displacement from the equilibrium position.

Wavelength (λ)

Distance between two identical points on adjacent wave cycles.

Frequency (f)

Number of wave cycles that pass a point in one second.

Period (T)

Time taken for one complete wave cycle.

Wave speed (v)

Speed at which the wave travels: v = fλ.

Reflection

When a wave bounces off a surface or boundary.

Refraction

When a wave changes speed and direction at a boundary between two media.

Diffraction

The spreading out of waves as they pass through a gap or around a barrier.

Superposition

When two or more waves meet, their displacements add together.

Constructive interference

When waves superpose to produce a larger amplitude.

Destructive interference

When waves superpose and cancel each other out.

Node

A point on a stationary wave with zero displacement.

Antinode

A point on a stationary wave with maximum displacement.

Critical angle

The minimum angle of incidence for total internal reflection to occur.

Total internal reflection

When all the wave is reflected at a boundary instead of being refracted.

Fringe spacing

The distance between two adjacent bright or dark fringes in an interference pattern.

Diffraction grating

An optical device with many slits that diffract light to produce an interference pattern.