Waves

A wave is caused by a vibrating source, and travels outwards from the source.

The particles of the medium through which a wave travels move about their equilibrium positions; they do not move along with the wave.

Energy is transferred from place to place by a wave.

A wave is caused by a vibrating source, and travels outwards from the source.

The particles of the medium through which a wave travels move about their equilibrium positions; they do not move along with the wave.

Energy is transferred from place to place by a wave.

Slinky Waves

To think about• Does the size of a pulse change as it travels along

the spring? • What is there was no friction?• What happens when a pulse reflects at the fixed

end? (we need to clamp the slinky at one end)• How does the speed change if you change the

tension? (does not work well, should be the same)

• What happens if pulses set off from opposite ends at the same time?

What you are going to achieve today• By the end of the lesson you will know

• What are the differences between transverse and longitudinal waves?

• What is a plane-polarised wave?• What physical test can distinguish transverse waves from

longitudinal waves?• What is meant by the amplitude of a wave?• Between which two points can the wavelength be

measured?• How is the frequency of a wave calculated from its period?• What is phase?

Important wave diagram

Radians and waves

Phase

Dual pendulums to demo phase

Phase Demo

Phase on the oscilloscope

V = f λ

V = velocity (m/s)f = frequency (how many waves in a second)λ = wavelength (m)

λ

c = f λ is used for electromagnetic waves as the velocity of light is always 3 x 108 m/s

All go at the speed of light

Speed, wavelength and frequency1)Light travels 300,000 km in one second.a) How long does it take light to travel 300 m? b) How long does it take light to travel 300 mm? 2)The speed of sound in a metal is 1000 m s–1.a.) What is the wavelength if the frequency is 2 kHz? b.) What is the wavelength if the frequency is 2 MHz?3)What is the frequency of sound waves of wavelength 100 mm? 4)What is the frequency of radio waves of wavelength 100 mm? 5)If you could 'freeze' ultrasound of wavelength 5 mm as it travels through tissue,

how many waves would there be in a one metre length? This quantity is called the wave number. How is it related to wavelength?

6)If a thunder clap comes 1.5 s after the lightning strike, how far away is the strike? Explain your calculation.

The speed of sound in air 330 m/s speed of light 3 x 108 m/s.

s = v ∆t

Answers

1) 1 microsecond b) 1 nanosecond2)

3) 4)

5) 200 waves per metre; the wave number is the reciprocal of the wavelength.

m 5.0Hz 2000

s m 1000 –1

f

cmm 5.0

Hz 102

s m 10006

–1

f

c

kHz 3.3m 10001

s m 3033–

–1

c

f

GHz 3m 10001

s m 1033–

–18

c

f

6) s = v ∆t = 330 x 1.5 = 495 m, light transit time is negligible

PolarisationPolarisation can only happen with transverse waves

http://www.youtube.com/watch?v=qlRriWu1zOI&feature=related

http://www.youtube.com/watch?v=ozTH2zIKwC0&feature=player_embedded

3D glasses

The water tubswaves in another medium

• The speed of a water wave

Create a water wave at one end of the tub andTime it to the other end

Change the depth of waterThere should be a difference but it is very fastSpeed = √g x depth (metres)

Wave height (from trough to crest),Wavelength (from crest to crest),Period (time interval between arrival of consecutive crests at a

stationary point),The direction of wave propagation.

CHALLENGE! Demonstrate longitudinal and transverse waves

In a group of 5 demo both types of wave