WavesWave characteristics

Travelling Waves

There are two types of mechanical waves and pulses that we encounter in

the physical world.

Transverse In these waves the source that

produces the wave oscillates at right angles to the direction of travel of the wave

It means that the particles of the medium through which the wave travels also oscillates at right angle to the direction of travel of the wave.

Direction of travelof the wave

Direction of oscillationof the particles

Transverse Wave

Longitudinal In these waves the source that

produces the wave oscillates in the same direction as the direction of travel of the wave

It means that the particle of the medium through which the wave travels also oscillates in the same direction as the direction of travel of the wave.

Longitudinal Wave

Direction of travelof the wave

Direction of oscillationsof the particles

Transverse Waves

Longitudinal Waves

What is a Wave?

A wave is a means by which energy is transferred between two points in a medium without any net transfer of the medium itself.

The Medium

The substance or object in which the wave is travelling.

When a wave travels in a medium parts of the medium do not end up at different places.

The energy of the source of the wave is carried to different parts of the medium by the wave.

Water waves however, can be a bit disconcerting;

Waves at sea do not transport water but the tides do.

Similarly, a wave on a lake does not transport water but water can actually be blown along by the wind.

Displacement

(d) is the distance that any particle is away from its equilibrium position at any given time.

Measured in meters

Amplitude

(A, a) This is the maximum displacement of a particle from its equilibrium position.

(It is also equal to the maximum displacement of the source that produces the wave).

Normally measured in meters

Period

(T) This is the time that it takes a particle to make one complete oscillation.

(It also equals the time for the source of the wave to make one complete oscillation).

Measured in seconds

Frequency

(f) This is the number of oscillations made per second by a particle.

(It is also equal to the number of oscillations made per second by the source of the wave)

The SI unit of frequency is the hertz -Hz. (1 Hz is 1 oscillation per second)

So, f = 1/T

Wavelength

() This is the distance along the medium between two successive particles that have the same displacement and the same phase of motion.

Ex. Distance between two consecutive wave crests.

Measured in meters

Wave Speed

(v, c) This is the speed with which energy is carried in the medium by the wave.

Measured in m/s A very important fact is that wave

speed depends only on the nature and properties of the medium.

Crest

This is a term coined from water waves and refers to the points at the maximum height of the wave.

Trough

A term coined from water waves referring to the points at the lowest part of the wave.

Wavelength again!

Wavelength will therefore be equal to the distance between successive crests and/or successive troughs.

Compression

This is a term used in connection with longitudinal wave and refers to the region where the particles of the medium are "bunched up".

High density High pressure

Rarefaction

A term used in connection with longitudinal waves referring to the regions where the particles are "stretched out".

Low density Low pressure

Longitudinal Waves The wavelength will be equal to the

distance between successive points of maximum compression and/or successive points of maximum rarefaction.

The compression is the region in which the molecules of the air are pushed together.

The rarefaction is the region where the molecules move apart.

rarefactions

wavelength

Sound Waves

A longitudinal wave in a slinky spring is analogous to a sound wave in which each turn of the spring represents an air molecule.

Interpreting Graphs - 1

displacement

distance

crest

trough

amplitude crestwavelength

amplitude

wavelength

Interpreting Graphs - 2

displacement

time

amplitudeperiod

period

Deriving v = f Imagine a wave with velocity v Being produced from a source of

frequency f In 1 second the 1st wavefront would

have travelled a distance of f As speed = distance / time v = f / 1 v = f

2 Important Points

1. The frequency of a wave depends only on the source producing the wave.

It will therefore not change if the wave enters a different medium or the properties of the medium change

2. The Speed of waves only depends on the nature and the properties of the medium.

Water waves do travel faster in deeper water

Light waves do travel slower in more optically dense material.

Sound waves do travel slower in cold air

The EM Spectrum Itself

Short Long High fLow f

VISIBLERadioWaves

MicroWaves

Infrared

Gammarays

UltraViolet

Xrays

Frequencies of Regions (Hz)

• Gamma Rays >1021

• X-rays 1017- 1021

• Ultraviolet 1014 - 1017

• Violet 7.5 x 1014 > Visible > Red 4.3 x 1014

• Infrared 1011 -1014

• Microwaves 109 -1011

• Radio and TV < 109

The Different Regions In the context of wave motion, common

properties of all parts of the electromagnetic spectrum are

all transverse waves all travel at the speed of light in vacuum

(3.0 x 108 m/s) all can travel in a vacuum

Sources of Regions Gamma – certain radioactive material’s nuclei X-rays – by firing an electron stream at a tungsten

metal target in a highly evacuated tube. Ultraviolet – the Sun, ultraviolet lamp Visible – hot bodies Infrared – the Sun (heat), hot bodies Microwaves – Ovens, communication systems Radio and TV – transmitter stations, Azteca TV