Lesson ObjectivesBy the end of this lesson, you will

be able to:

• State the different types of waves and the difference between them

• Describe the diffraction of light

• Explain how to work out the speed of wave propagation

• Waves transfer energy & information but without transferring matter

• The energy is transferred by oscillations in the material which the wave is travelling though

1) Mechanical Waves

Waves that pass though a material are vibrations of that material

eg. Sound waves, seismic waves, strings

2) Electromagnetic Waves

Vibrating electrical or magnetic fields through space (no material needed)

eg electromagnetic spectrum

1. Longitudinal Waves

• Oscillations occur parallel to direction of travel

• Sound waves – a vibrating surface in contact with air.

• The surface pushes air molecules away which push adjacent air molecules

Compression (C)

Expansion (E)

E E EC C CDirection

of Travel

which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules which push adjacent air molecules…….

2. Transverse Wave

Direction of

Travel

• Oscillations at right angles (90o) perpendicular to direction of travel

• Electromagnetic waves – radiowaves, X-rays, microwaves, visible light etc

• Waves on a string or wire – plucking a guitar string

• Can only polarise transverse waves

• When an earthquake occurs, a majority of the energy is transferred as wave energy and transmitted over long distances.

• Classified in 2 types

1) P primary or push waves

2) S secondary or shake/shear waves

A

Parts of a Wave

x

Wavelength

Peak

Amplitude

Trough

1 complete wave

Key Terms• Displacement (s) – the distance from the

equilibrium position

• Wavelength () – the distance between identical points in a wave train

• Amplitude (A) – maximum displacement of a particle (peak or trough max.)

Key Terms• Period (T) – the time taken for 1 complete

wave to pass a point

• Frequency (f) – number of cycles per second. Measured in Hertz (Hz).

fT

1

Amplitude and FrequencyLow amplitude, low frequency:

Low amplitude, high frequency:

High amplitude, low frequency:

High amplitude, high frequency:

WAVE SPEED• We know the relationship between

distance, speed and time.• Speed = Distance / Time• Therefore…

D

TS

Wave Speed = Wavelength / Period

V = λ / T

But we know that T = 1/f

So we can substitute….

The Wave Equationrelates the speed of the wave to its frequency and wavelength

Wave speed (v) = frequency (f) x wavelength ()

m/s Hz m

V

f

Some example wave equation questions

1) The speed of sound is 330m/s (in air). When Cuthbert hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound?

2) a) Purple light has a wavelength of around 6x10-7m. If its frequency is 5x1014 Hz. What is the speed of light?

b) Red light travels at the same speed. Work out its frequency if its wavelength is about 4x10-7m.

TSUNAMIa) A typical tsunami has a wavelength of 300km and a period

of 30 mins. What speed does it travel at?

b) Brighton is 2800km from La Palma – how long do we have to evacuate before it hits?

1. Reflection

Waves will bounce off a surface under certain conditions

eg the surface must be shiny for electromagnetic waves

Reflective surface

Incident Ray

Reflected Ray

Normal

Angle of Incidence

Angle of Reflection

Angle of Incidence = Angle of Reflection

2. Refraction

Glass Block

Air

Waves cross a boundary causing a change in speed and consequently wavelength

Depends on the refractive index of different substancesNormal

Normal

3. Diffraction

• Occurs when waves pass through a gap or around an object of roughly the same size or smaller than their wavelength.

•Large gap - the middle parts of the waves go straight through the gap, with a slight curving at the edges of the waves.

•Small gap - if the gap is smaller than the wavelength of the waves, the waves fan out in circles.

1. Constructive Interference - when the crests (or troughs) of two waves coincide, they combine to create an amplified wave.

The two waves are in phase with each other – there is zero phase difference between them.

2. Destructive Interference - where the crests of one wave are aligned with the troughs of another, they cancel each other out.

The waves are out of phase (or in antiphase) with each other – they are half a cycle different from each other.

Lesson ObjectivesBy the end of this lesson, you will

be able to:

• State the different types of waves and the difference between them

• Describe the diffraction of light

• Explain how to work out the speed of wave propagation

GOOD WORK HUMANS!