HC

The Professional Development Service for Teachers is funded by the Department of Education and Science under the National Development Plan

Waves and wave motion

Can you think of some examples of waves?

Waves

•Sound

•Light

•Water

•Radio

•Seismic.

What is the function of a wave?

A wave is a means of transferring energy from one place to another

TYPES OF WAVESTYPES OF WAVES

T ra ns ve rs e w a ves L o n g itu d ina l w a ves

Mechanical w aves

T ra ns ve rs e w a ves

Elec tromagnetic w aves

WA V E S

Waves Waves are classified into different types according to their natures :

Wave Types

Mechanical Electromagnetice.g. Water waves, waves on a rope, sound

e.g. Radio, X-Rays, Light

Must have a substance to travel through

Cannot travel in a vacuum

Does not need a substance to travel through but can travel through various substances

Can travel in a vacuum

Vibrations passed on from molecule to molecule

Travel at very fast speed in a vacuum: 3 × 108 m/s

Travelling Waves

Waves that travel from one place to another

e.g. Waves on rope e.g Waves on water e.g. compression

waves on spring

Compression and Rarefraction

Longitudinal wave

Types Of Waves

A wave where the direction of the vibrationA wave where the direction of the vibrationis is perpendicularperpendicular to to

the direction in which the wave travelsthe direction in which the wave travels

TRANSVERSETRANSVERSE

LONGITUDINALLONGITUDINAL

A wave where the direction of the vibrationA wave where the direction of the vibrationis is parallelparallel to the to the

Direction in which the wave travels.Direction in which the wave travels.

Transverse Waves

Transverse waves A transverse wave is a one in which the

direction of vibration is perpendicular to the direction of propagation.

Examples Light waves. Radio waves. Waves on a rope. Water waves

Longitudinal Waves

Longitudinal Waves A longitudinal wave is one in which the

direction of vibration is parallel to the direction of propagation.

Examples Sound waves in a solid, liquid or gas. Compression waves on a spring. Seismic waves.

Wave..........

The wavelength of a wave is the distance from one point on the wave to the corresponding point on the next cycle.

The frequency of a wave is a measure of the number of oscillations (vibrations) of the wave per second.

The periodic time of a wave (T) is the time taken for one complete cycle.

Symbols and units

Variable Symbol Unit Symbol for Unit

Frequency f Hertz Hz

Wavelength (“lamda”)

metre m

Velocity v (or c for light)

metre/second m/s

Time T second s

The relationship between frequency, velocity and

wavelength

c = f or

v = f

Properties of waves

1. Reflection: Reflection is the bouncing of a wave off an object.

Properties of waves

2 Refraction is the bending of a wave as it travels from one medium to another. Note that when a wave travels from one medium to another its frequency does not change.

Properties of waves

3. Diffraction is the spreading of waves around a slit or an obstacle.

This effect is only significantly noticeable if the slit width is approximately the same size as the wavelength of the waves.

Properties of waves

4 Interference occurs when waves from two sources meet to produce a wave of different amplitude.

Interference of WavesInterference of Waves

When two or more waves propagating in the same medium meet at the same point, interference interference is said to occur.

A stable interference pattern can be observed when two water waves of same frequency meet one another in a ripple tank.

+=

Two types of interference

Constructive Interference Destructive Interference

Constructive Interference

Constructive Interference occurs when waves from two coherent sources meet to produce a wave of greater amplitude.

(Constructive interference occurs when the crests of one wave are over the crests of another wave).

Destructive Interference

Destructive Interference occurs when waves from two coherent sources meet to produce a wave of lower amplitude.

(Destructive interference occurs when the crests of one wave are over the troughs of the second wave. This will happen if one wave is half a wavelength out of phase with respect to the other).

Only TRANSVERSE waves can be polarised.

To polarise a wave means to make it vibrate in one plane only–horizontally plane polarised or

–vertically plane polarised

5. Polarisation

Polarised sunglasses

The frequency of a wave is a measure of the number of oscillations (vibrations) of the

wave per second.

Another way of defining frequency of a wave is to say it is the number of waves that pass a

fixed point per second. T = 1/f

The relationship between periodic time and frequency

T = 1/f

or

f = 1/T

A standing wave/stationary wavestanding wave/stationary wave is theresult of two waves of the same

wavelength, frequency, and amplitudetravelling in opposite directions

through the same medium.

Standing waves

From the diagram we can see that:1. The distance between two

consecutives nodes is /22. The distance between two

consecutive antinodes is /23. The distance between an anti-

node and the next node is /4

• The Doppler effect is the apparent change in frequency of a wave due to the motion of the source or the observer.

• The observed frequency is higher when the source and observer are getting closer.

• The observed frequency is lower when the source and observer are getting farther away.

The Doppler EffectThe Doppler Effect

• The distance between successive crests is the constant and so the number of crests passing a point in one second is the frequency of the wave.

• These waves will pass an observer in equal intervals of time.

• This means that the wavelength, and the frequency, will be constant.

Consider a source S emitting a wave with

crests 1, 2, 3 as shown.

In this case the source is moving to the right while emitting the waves.

The result is that:• Ahead of the moving source

the crests are closer together than crests from the stationary source would be. This means that the wavelength is smaller and the frequency is greater.

• Behind the moving source, the crests are further apart than crests from the stationery source would be. This means the wavelength is greater and so the frequency is less.

Formula: f’ = apparent frequencyf = actual frequencyc = speed of the waveu = speed of the moving source

uc

fcf

The noise from a racing car as it approaches and then moves away from an observer is an example of the Doppler effect.

This is NOT an APPLICATION of the Doppler effect.

Example of the Doppler Effect

• Police speed traps

• Measuring the red shift of galaxies in astronomy

Applications of the Doppler Effect

Radar

True Velocity

Tangential Velocity

Radial Velocity

Doppler shift gives radial velocityDoppler shift gives radial velocity