AQA Physics A-Level - Section 3: Waves

Comprehensive vocabulary flashcards covering wave parameters, progressive and stationary waves, polarisation, interference, diffraction, refraction, total internal reflection, and optical fibre principles for AQA A-Level Physics.

Progressive wave

A wave that transfers energy without transferring material, made up of oscillating particles of a medium or oscillating fields.

Amplitude

A wave's maximum displacement from the equilibrium position (units are $\text{m}$).

Frequency ($f$)

The number of complete oscillations passing through a point per second (units are $\text{Hz}$).

Wavelength ($\lambda$)

The length of one whole oscillation, such as the distance between successive peaks or troughs (units are $\text{m}$).

Wave speed ($c$)

The distance travelled by the wave per unit time (units are $\text{m/s}$), calculated as $c = f\lambda$.

Phase

The position of a certain point on a wave cycle (units are radians, degrees, or fractions of a cycle).

Phase difference

How much a particle or wave lags behind another particle or wave (units are radians, degrees, or fractions of a cycle).

Period ($T$)

The time taken for one full oscillation (units are $\text{s}$), related to frequency by $f = \frac{1}{T}$.

In phase

Describes two points on a wave that are at the exact same point of the wave cycle, having the same displacement and velocity with a phase difference that is a multiple of $360^\circ$ ($2\pi\,\text{radians}$).

Completely out of phase

Describes two points on a wave that are an odd integer of half cycles apart (e.g. $5$ half cycles apart), where one half cycle corresponds to $180^\circ$ ($\pi\,\text{radians}$).

Transverse wave

A wave in which the oscillation of particles (or fields) is at right angles to the direction of energy transfer (e.g. all electromagnetic waves, which travel at $3 \times 10^8\,\text{ms}^{-1}$ in a vacuum).

Longitudinal wave

A wave in which the oscillation of particles is parallel to the direction of energy transfer, composed of compressions and rarefactions, and unable to travel through a vacuum.

Polarised wave

A wave that oscillates in only one single plane (e.g. only up and down). Only transverse waves can be polarised.

Superposition

The phenomenon where the displacements of two waves combine as they pass each other, producing a resultant displacement equal to the vector sum of each wave's individual displacement.

Constructive interference

A type of interference that occurs when two waves superpose with displacement in the same direction, creating a larger resultant displacement.

Destructive interference

A type of interference that occurs when one wave has positive displacement and the other has negative displacement, cancelling each other out completely if the displacements are equal and opposite.

Stationary wave

A wave formed by the superposition of two progressive waves travelling in opposite directions in the same plane with equal frequency, wavelength, and amplitude, which transfers no energy.

Antinode

A region on a stationary wave with maximum amplitude, formed where progressive waves meet in phase and interfere constructively.

Node

A region on a stationary wave with no displacement, formed where progressive waves meet completely out of phase and interfere destructively.

First harmonic

The lowest frequency at which a stationary wave forms, characterized by two nodes and a single antinode, calculated using the formula $f = \frac{1}{2L}\sqrt{\frac{T}{\mu}}$.

Path difference

The difference in the distance travelled by two individual waves to reach a specific point.

Coherent light source

A light source emitting waves that have the same frequency and wavelength as well as a fixed phase difference.

Monochromatic light

Light that consists of a single (or very small range of) wavelength(s).

Young's double slit experiment

An experiment demonstrating light interference by passing light through two slits of width comparable to the wavelength, producing alternating bright fringes ($n\lambda$) and dark fringes (($n + \frac{1}{2})\lambda$).

Fringe spacing ($w$)

The distance between adjacent fringes in a double slit interference pattern, calculated using $w = \frac{\lambda D}{s}$, where $D$ is the distance to the screen and $s$ is slit separation.

Diffraction

The spreading out of waves when they pass through a gap or around an obstacle, which is greatest when the gap is the same size as the wavelength.

Single slit diffraction pattern

An intensity pattern consisting of a bright central fringe double the width of all other fringes, flanked by alternating dark and bright fringes of decreasing intensity.

Diffraction grating

A slide containing many equally spaced slits positioned very close together, producing a much sharper and brighter interference pattern than a double slit.

Diffraction grating equation

The formula $d\sin(\theta) = n\lambda$, where $d$ is the slit spacing, $\theta$ is the angle from the normal to the maximum, $n$ is the order number, and $\lambda$ is the wavelength.

Refractive index ($n$)

A property of a material measuring how much it slows down light passing through it, calculated by $n = \frac{c}{c_s}$, where $c$ is the speed of light in a vacuum and $c_s$ is the speed of light in the substance.

Snell's law

The law governing light refraction at a boundary: $n_1\sin(\theta_1) = n_2\sin(\theta_2)$, relating the refractive indices and the angles of incidence and refraction.

Critical angle ($\theta_c$)

The angle of incidence at which the angle of refraction equals exactly $90^\circ$, calculated using $\sin(\theta_c) = \frac{n_2}{n_1}$ where $n_1 > n_2$.

Total internal reflection (TIR)

The complete reflection of a light ray within an optically dense medium when the angle of incidence is greater than the critical angle ($\theta_1 > \theta_c$) and $n_1 > n_2$.

Optical fibre

A thin, flexible tube of plastic or glass designed to guide light via total internal reflection, comprising an optically dense core surrounded by cladding of lower optical density.

Modal dispersion

A form of dispersion caused by light rays entering an optical fibre at different angles and traversing different path lengths, leading to pulse broadening; reduced by using a very narrow core.

Material dispersion

A form of dispersion caused by light composed of different wavelengths travelling at different speeds through an optical fibre, leading to pulse broadening; prevented by using monochromatic light.

Absorption (optical fibres)

The reduction in signal amplitude caused by the fibre material absorbing part of the signal's energy, which can lead to information loss.

Pulse broadening

An effect caused by dispersion where the received signal is broader in time than the original transmitted signal, potentially causing overlapping pulses and loss of information.