Topic 4 Oscillations and Waves

4.3 Wave Characteristics

Waves

A wave is an oscillation that can move energy from place to place without the transfer of matter.

An oscillation moves no energy nor matter overall.

Waves can either be a pulse or a continuous travelling wave.

In most cases the oscillations of the particles making up the wave can be considered to be simple harmonic motion.

Types of waves

Transverse wavesThe vibration of the particles are at 90o to the direction of propagation of the wave.

These are seen as water waves, surface earthquake waves, and as electromagnetic waves.

Physical trasverse waves cannot propagate in gases.

Longitudinal wavesThe vibration of the particles is parallel to the direction of propagation of the wave.

These are pressure waves and are seen as sound waves and earthquakes.

Transverse Waves

Crest

Trough

Amplitude

Wavelength

x

r

Oscillations

Energy

Displacement

Longitudinal Waves

Compression

Rarefaction

Amplitude

Wavelength

y

x, r

Oscillations

Energy

Displacement

Intensity and Amplitude

The intensity of a wave is the energy delivered by it per second per unit area of detector.

It therefore has units of Wm-2

The intensity of a wave is directly proportional to the amplitude of the wave squared.

Frequency and Period

The period (T) of a wave is the time taken in seconds for it to make one complete cycle.

As most of the waves that we use have very short periods we often use the frequency of the wave instead.

The frequency (f) of a wave is the number of complete cycles in 1 second and is measured in hertz (Hz).

Wave Speed

The speed with which a wave transfers energy from place to place is known as the wave speed.

The distance a wave travels in one cycle is the wavelength.

The time taken for one oscillation is the time period.

The wave speed is therefore given by:

Waves in 2 Dimensions

There are two distinct ways to describe the motion of waves in 2 dimensionsRays The wave is represented by an arrow which lies in the direction of propagation.

Wavefronts The wave is represented by a series of parallel lines that are perpendicular the direction of propagation and show the position of each wave crest or compression.

Both of these methods have their uses in specific situations and can sometimes be seen used together.

Waves in 2 Dimensions

A series of wavefronts

A light ray

Combined notation

The same wave in 2 dimensions

Transverse Waves in 2 Dimensions

Sometimes, the direction of oscillation of a transverse wave is shown on the ray.

Oscillations Up and Down

Oscillations In and Out

Oscillations both Up and Downand In and Out

Electromagnetic Waves

Electromagnetic waves rely on the oscillations of electric and magnetic fields rather than particles to move energy.

In a vacuum, all electromagnetic waves travel at the same speed, c = 3.00x108 ms-1

Electromagnetic Spectrum

Electromagnetic waves can have any frequency.

The continuous spectrum of electromagnetic waves goes from radio waves at low energy up to high energy gamma waves

Different wave bands are divided by their wavelengths

The Electromagnetic Spectrum

Questions

An electromagnetic wave has a frequency of 5.6x1012 Hz. What is the wavelength and waveband of this radiation?

High energy x-rays have a wavelength of 10pm. What is the frequency of this radiation in a vacuum?

A red laser is fired in a vacuum. What is the frequency of this radiation.

A sound wave is measured to have a frequency of 15kHz and a wavelength of 2.2cm. What is the speed of this sound?