Waves basics

Covers longitudinal and transverse waves, polarisation and other basic definitions.

What is a progressive wave?

A wave that transfers energy without transferring material.

What is the amplitude of a wave?

A wave’s maximum displacement from the equilibrium position (in metres)

What is the frequency of a wave?

The number of complete oscillations passing through a point per second (in Hertz)

What is the wavelength of a wave?

The length of one whole oscillation (i.e. peak-peak/trough-trough) (in metres)

What is the wavespeed?

Distance travelled by the wave per unit time (usually m/s)

What is the phase of a wave?

The position of a certain point on a wave (in radians/degrees)

What is the meant by the phase difference of two waves?

How much a wave lags behind another wave (usually radians/degrees)

What is the time period of a wave?

The time taken for one full oscillation to pass one point. (in seconds)

What does it mean when two waves are in phase?

When both waves are at the same point in the wave cycle. They have the same displacement and velocity, and the phase difference is a multiple of 360° or 2π rad. They need the same frequency and wavelength.

What does it mean when two waves are in anti-phase?

When two waves are at opposite phases i.e 180° or πrad out of phase.

What is a longitudinal wave?

A wave which oscillates in parallel to the direction of energy transfer. They are made up of compressions and rarefactions, and cannot travel through a vacuum.

Sound is longitudinal.

What is a transverse wave? Are EM waves transverse?

A wave which oscillates perpendicular to the direction of energy transfer. They have peaks and troughs and can travel in a vacuum.

All electromagnetic waves are transverse.

Longitudinal waves can be drawn to have peaks and troughs, so not a defining characteristic.

What is a polarised wave?

A wave that only oscillates in one plane e.g. up and down or left and right. This only happens to transverse waves.

What is the condition for polarisation of a wave?

Only occurs if a wave’s oscillations are perpendicular to its direction of travel

Give some examples of polarisation.

Windshields, sunglasses, tv signals, radio signals.

Explain how polarisation works with a car windshield reducing glare on a road.

Light from the sun is reflected off the road, which creates glare. This light is partially polarised. The windscreen is polarised horizontally in order to prevent the polarised waves from the road passing through to the driver.

Explain how polarisation works with radio signals.

(This is applicable to tv too)

Radio signals are transmitted in a particular polarisation. The spokes on an antenna receiving a radio signal can be orientated vertically or horizontally in order to match the transmitted signal polarisation. Therefore only that signal can be picked up.