Waves

GCSE Physics

Objectives of the Lesson

• Understand the concept of wave motion in the physical world

• Recall that there are two types of wave motion- Longitudinal and Transverse

• Explain the difference between the types of waves and use the correct terms for each

• State some examples of each of the types of wave motion

Waves Introduction

• A wave can be described as a disturbance that travels through a medium from one place to another.

• Medium – a substance which something is transmitted or carried through e.g.. air

Examples of waves

• Sea wave (medium- liquid, water)

• Shotgun blast (medium- gas, air)

Types of Waves- TransverseWave motion vs Particle motion

Watch one particle

Describe its motion

Transverse wave…

In terms of the wavelength, what is the length of the red line on the graph?

Wavelength λ – distance from one point on one wave to the same point on the next wave (for example- from crest to crest)

CrestCrest CrestCrest

TroughTrough TroughTrough

Wavelength λWave spreadsWave spreads

Wave movementWave movement

Light travels as a transverse wave

Wave movement is at right angles to the direction that the wave is travelling

Amplitude – maximum displacement from its undisturbed position

Amplitude

Types of Waves- Longitudinal Wave motion vs Particle motion

Keep your eye on 1 particle

Describe its motion

If the diagram shows the motion of the wave in 1 second then what is the frequency of the wave?

Frequency – the number of waves passing a point every second (Hz)

Longitudinal wave

Rarefaction Compression

Sound waves travel as longitudinal waves

Wave movement is parallel to the direction the wave is travelling

Water Waves

• Water waves are an example of waves that involve a combination of both longitudinal and transverse motions. As a wave travels through the water, the particles travel in clockwise circles.

Longitudinal Vs Transverse

• Create a table with two column headings, Longitudinal and Transverse

Wave motion is at right angles to the

direction of the wave

Wave motion is parallel to the direction of the wave

Amplitude is maximum displacement from the wave’s rest position

Frequency is the number of waves

passing a point in one second

Wavelength is the distance from one crest to the next

crest

Wavelength is the distance from one compression to the next compression

This is how sound energy moves

This is how light travels

Graph action…

http://surendranath.tripod.com/Applets/Waves/Twave01/Twave01Applet.html

Objectives of the Lesson

• Recall the wave equation and use it to solve simple problems

• Describe, using simple wavefront diagrams, how plane waves are reflected at a plane barrier, refracted at a plane boundary

The Wave Equation

• Speed (v) = Frequency (f) x Wavelength (λ)

m/s Hz m

Example

• What is the speed of a water wave of frequency 4 Hz and wavelength 3 cm?

v = f x λ

= 4 x (3 / 100)

= 0.12 m/s

Try this one…

What is the wavelength of a sound wave of frequency 264 Hz and speed 330 m/s?

v = f x λ

330 = 264 x λ

λ = 330 / 264 = 1.25 m

What is the distance between one compression and the next rarefraction on this wave?

1.25 / 2 = 0.625 m

Question Time

• Pg 105 Question 1 a) parts i) – iv)

Objectives of the Lesson

• Describe, using simple wavefront diagrams, how plane waves are reflected at a plane barrier, refracted at a plane boundary

Ripple Tank

Wave motion

Crest

Trough

Wave Diagrams

• Law of Reflection- The angle of incidence is always equal to the angle of reflection

Wave front Wave front (crest)(crest)

The angle the wave hits the barrier at is 23 degrees.

1. What is the angle of incidence?

2. Draw the three waves after they have hit the

barrier and label the angle of reflection and include the

normal line.

Example

23º

Normal line

Angle of incidence 67º

Angle of Angle of reflection 67reflection 67ºº

Objectives of the Lesson

• Describe, using simple wavefront diagrams, how plane waves are reflected at a plane barrier, refracted at a plane boundary

Shallow Shallow waterwater

Deep water

• RefractionRefraction-- For a wave moving from deep to shallow, the wavelength will shorten because the wave slows down.

• The frequencyfrequency of the wave stays the same

Refraction

Shorter Shorter Wavelength- Wavelength- waves speed waves speed slowerslower

Shallow Shallow waterwater

Deep water

λ

λ

wavefront

Refraction• If the incident wave travels at an angle and

meets shallow water, then the wave will be refracted (shifted) towards the normal

Angle of Angle of refractionrefraction

Angle of incidenceAngle of incidence

Normal line

Shallow water

Deep water

Angle of incidence Angle of refractionAngle of refraction>

Refraction

• What do you think will happen when a wave travels into deeper water? Draw a diagram with labels to represent the situation.