Waves and wave motion

HC

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

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 WAVES

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

Wave Types

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 vibration

is perpendicular to

the direction in which the wave travels

TRANSVERSE

LONGITUDINAL

A wave where the direction of the vibration

is parallel to the

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

The relationship between frequency, velocity and wavelength�

c = f λ

or

v = f λ

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 Waves

When two or more waves propagating in the same medium

meet at the same point, 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).

5. Polarisation

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

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�

Standing waves

A standing wave/stationary wave is the

result of two waves of the same

wavelength, frequency, and amplitude

travelling in opposite directions

through the same medium.

From the diagram we can see that:

1. The distance between two consecutives nodes is λ/2
2. The distance between two consecutive antinodes is λ/2
3. The distance between an anti-node and the next node is λ/4

The Doppler Effect

• 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 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 frequency

f = actual frequency

c = speed of the wave

u = speed of the moving source

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

Applications of the Doppler Effect

• Police speed traps �
• Measuring the red shift of galaxies in astronomy

​

​

Doppler shift gives radial velocity