1. OCR Gateway 2016 Physics topic 5
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21/09/2018
Waves in Matter
OCR Gateway 2016 Physics topic 5
W Richards
The Weald School

2. P5.1 Wave Behaviour
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3. Some definitions… 1) Amplitude – this is “how high” the wave is:
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1) Amplitude – this is “how high” the wave is:
2) Wavelength () – this is the distance between two corresponding points on the wave and is measured in metres:
3) Frequency – this is how many waves pass by every second and is measured in Hertz (Hz).
By definition, frequency = 1/period (in s).

4. Wave speed (v) = frequency (f) x wavelength ()
The Wave Equation
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The wave equation relates the speed of the wave to its frequency and wavelength:
Wave speed (v) = frequency (f) x wavelength ()
in ms in Hz in m V  f

5. Some example wave equation questions
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A water wave has a frequency of 2Hz and a wavelength of 0.3m. How fast is it moving?
A water wave travels through a pond with a speed of 1ms-1 and a frequency of 5Hz. What is the wavelength of the waves?
The speed of sound is 330ms-1 (in air). When Dave hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound?
Purple light has a wavelength of around 6x10-7m and a frequency of 5x1014Hz. What is the speed of purple light?
0.6ms-1
0.2m
0.5m
3x108ms-1

6. Transverse vs. longitudinal waves
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Displacement
Transverse waves are when the displacement is at right angles to the direction of the wave…
Direction
Displacement
Longitudinal waves are when the displacement is parallel to the direction of the wave…
Direction
Where are the compressions (areas of high pressure) and rarefactions (areas of low pressure)?

7. An example question V/mV What type of wave is this?
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V/mV
What type of wave is this?
What is the wave’s:
Amplitude
Time period
Frequency? 3 2 1 1 2 3 4 5 6 7 8
T/ms -1 -2

8. What happens when a wave hits a boundary?
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When a wave hits a boundary a number of things could happen:
1) It could be reflected
2) It could be transmitted or refracted through the material
3) It could be absorbed
Note that the frequency of the waves doesn’t change. However, the velocity and wavelength might!

9. Angle of incidence = Angle of reflection
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Angle of incidence = Angle of reflection
Normal
Incident ray
Reflected ray
Angle of incidence
Angle of reflection
Mirror

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Refraction

11. Refraction through a glass block
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Light slows down and bends towards the normal due to entering a more dense medium
Notice that waves can be transmitted and reflected at an interface:
Light slows down but is not bent, due to entering along the normal
Light speeds up and bends away from the normal due to entering a less dense medium

12. Examples of using waves
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Examples of using waves
Echo sounding
In echo-sounding, high frequency sound waves are reflected from an object to measure the distance to that object:

13. Words – depth, reflected, picture, medium
Example 2 - Ultrasound
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Ultrasound is the region of sound above 20,000Hz – it can’t be heard by humans. It can be used in pre-natal scanning:
How does it work?
Ultrasonic waves are partly _________ at the boundary as they pass from one _______ to another. The time taken for these reflections can be used to measure the _______ of the reflecting surface and this information is used to build up a __________ of the object.
Words – depth, reflected, picture, medium

14. Sound Waves (Physics higher tier only)
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As we know, sound waves are formed when something vibrates. But how does the sound reach our ears?
Air molecules
2) The vibrations pass through air by making air molecules vibrate
3) These vibrations are picked up by the ear drum and other parts of the ear, which also vibrate
1) An object makes a sound by vibrating

15. Converting Sound to Vibrations
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The ear drum
The ear bones
In a microphone the sound waves cause a diaphragm to move
Note that our ear drum and bones can only vibrate at a certain rate. Therefore our hearing is limited to between 20Hz and 20,000Hz. This is called our “range of hearing”.

16. Measuring the speed of waves
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Task:
Design and carry out two experiments that will help you to determine:
The speed of a ripple in water
The speed of sound in air

17. What moves when a wave travels?
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A wave is a “movement of energy”. Notice that the “stuff” the wave moves through doesn’t actually move from left to right, but energy does!

18. P5.2 The Electromagnetic Spectrum
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P5.2 The Electromagnetic Spectrum

19. Electromagnetic Radiation
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Here are some examples of electromagentic radiation being used to transfer energy:

20. The Electromagnetic Spectrum
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The Electromagnetic Spectrum is a continuous spectrum of transverse waves of different wavelength and frequency:
High frequency, _____ wavelength
Low frequency, _____ (high) wavelength
Gamma rays
X-rays
Ultra violet
Visible light
Infra red
Microwaves
Radio/TV γ
Each of these types travels at the same speed through a _______ (300,000,000m/s), and different wavelengths are absorbed by different surfaces (e.g. infra red is absorbed very well by ___________ surfaces). This absorption may heat the material up (like infra red and _______) or cause an alternating current (like in a __ _______). Our eyes can only detect a very small part of the spectrum called the _______ light region.
Words – black, microwaves, visible, long, short, TV aerial, vacuum

21. Visible Light
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Here’s a picture we’re all familiar with. What’s wrong with it?
In reality, the colours in a spectrum or rainbow change gradually, like this:
Red – wavelength m
Blue – wavelength m
Each of these colours has its own narrow band of wavelengths and frequencies. For example:

22. Using Infra Red waves Some examples:
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Some examples:
Remote controls – the signal from the remote contains a set of digital codes that carry different instructions to the device.
Passive infra red detectors that detect body heat

23. Using Satellites with microwaves
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Microwaves are used to communicate with satellites

24. The Electromagnetic Spectrum
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Type of radiation
Uses
Dangers
Treating cancer, sterilisation
Gamma rays
Cell mutation
X rays
Medical
Cell mutation
Ultra violet
Sun tanning
Skin cancer
Seeing things, fibre optic cables
None (unless you look at the sun)
Visible light
Remote controls, heat transfer
Infra red
Sunburn
Microwaves
Satellites, phones
Very few
TV/radio
Communications
Very few

25. Medical Imaging (HT only)
Radiation is safe, isn’t it?!?
There are three types of radiation on the Electromagnetic Spectrum that can cause humans harm:
1) Gamma radiation (an ionising radiation that causes mutation of cells and cancer)
2) X-rays (also cause cell mutation and cancer)
3) Ultra violet (causes premature aging of skin and skin cancer)
Differences in velocity, absorption and reflection of waves can be used to explore things inside the body, such as ultrasound or x-rays:

26. Using Radio Waves (Higher only)
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Radio waves can also be used as “carrier waves” in order to send a signal:
The radio turns the radio wave back into an electrical signal. The oscillating wave “induces” an oscillating electrical signal.
Radio waves are produced in a transmitter using oscillations
The wave is transmitted through the air

27. P5.3 Wave Interactions
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28. Recap: What happens when a wave hits a boundary? (HT only)
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Recap: What happens when a wave hits a boundary? (HT only)
When a wave hits a boundary a number of things could happen:
1) It could be reflected
2) It could be transmitted or refracted through the material
3) It could be absorbed
The same things can happen to an electromagnetic wave. Different effects may happen to different wavelengths and some effects depend on the speed of different waves.

29. Lenses Lenses use the idea of refraction:
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Lenses use the idea of refraction:
When light enters a MORE DENSE medium it slows down…
A prism uses this idea to split light. This happens because purple light is refracted more than red light

30. Converging and diverging lenses
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CONVERGING (Convex)
Thickest at the centre
DIVERGING (Concave)
Thinnest at the centre

31. Ray Diagrams for Lenses
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The rays of light are refracted INWARDS and meet at the focus, F.
The image formed is VIRTUAL or REAL – a real image can be seen on a screen whereas a virtual image can not. F
Focal length
The rays of light are refracted OUTWARDS.
A VIRTUAL image is always formed – in other words, the image doesn’t actually exist F
Focal length

32. Using lenses to correct vision
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Short
Long

33. What happens when light hits a material?
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Here are four possibilities. What are they called?
Glass
Wood X
Mirror
A rough surface
Absorption
Diffuse reflection
Specular reflection
Transmission

34. Investigating filters
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Task: Use your filters to determine which “colours” can be seen on the next slide. If you can’t read the work in the middle of the box then you can’t see the colour!
Colour of filter
Colours that could be “seen”
Red
Green
Blue
Cyan
Magenta
Yellow

35. Red Blue Green White Yellow Cyan Magenta
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Red
Blue
Green
White
Yellow
Cyan
Magenta

36. Using filters
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Filters can be used to “block” out different colours of light by absorbing those colours and allowing others to pass through:
Red Filter
Magenta Filter

37. Seeing colour revision
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The colour an object appears depends on the colours of light it reflects. The object will absorb the other colours.
For example, a red book only reflects red light and absorbs everything else:
Homework
White
light
Only red light is reflected

38. A white hat would reflect all seven colours:
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A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue):
Purple light
A white hat would reflect all seven colours:
White
light

39. Using coloured light
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If we look at a coloured object in coloured light we see something different. For example, consider a football kit:
Shirt looks red
White
light
Shorts look blue

40. In different colours of light this kit would look different:
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In different colours of light this kit would look different:
Red
light
Shirt looks red
Shorts look black
Shirt looks black
Blue
light
Shorts look blue

41. What colour would the following objects look like?
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Object
Colour of light
Colour object seems to be
Red socks
Red
Blue
Black
Green
Blue teddy
Green camel
Magenta book
Homework