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

P5.1 Wave Behaviour 21/09/2018

Some definitions… 1) Amplitude – this is “how high” the wave is: 21/09/2018 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).

Wave speed (v) = frequency (f) x wavelength () The Wave Equation 21/09/2018 The wave equation relates the speed of the wave to its frequency and wavelength: Wave speed (v) = frequency (f) x wavelength () in ms-1 in Hz in m V  f

Some example wave equation questions 21/09/2018 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

Transverse vs. longitudinal waves 21/09/2018 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)?

An example question V/mV What type of wave is this? 21/09/2018 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

What happens when a wave hits a boundary? 21/09/2018 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!

Angle of incidence = Angle of reflection 21/09/2018 Angle of incidence = Angle of reflection Normal Incident ray Reflected ray Angle of incidence Angle of reflection Mirror

21/09/2018 Refraction

Refraction through a glass block 21/09/2018 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

Examples of using waves 21/09/2018 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:

Words – depth, reflected, picture, medium Example 2 - Ultrasound 21/09/2018 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

Sound Waves (Physics higher tier only) 21/09/2018 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

Converting Sound to Vibrations 21/09/2018 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”.

Measuring the speed of waves 21/09/2018 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

What moves when a wave travels? 21/09/2018 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!

P5.2 The Electromagnetic Spectrum 21/09/2018 P5.2 The Electromagnetic Spectrum

Electromagnetic Radiation 21/09/2018 Here are some examples of electromagentic radiation being used to transfer energy:

The Electromagnetic Spectrum 21/09/2018 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

Visible Light 21/09/2018 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 0.0000007m Blue – wavelength 0.0000004m Each of these colours has its own narrow band of wavelengths and frequencies. For example:

Using Infra Red waves Some examples: 21/09/2018 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

Using Satellites with microwaves 21/09/2018 Microwaves are used to communicate with satellites

The Electromagnetic Spectrum 21/09/2018 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

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:

Using Radio Waves (Higher only) 21/09/2018 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

P5.3 Wave Interactions 21/09/2018

Recap: What happens when a wave hits a boundary? (HT only) 21/09/2018 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.

Lenses Lenses use the idea of refraction: 21/09/2018 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

Converging and diverging lenses 21/09/2018 CONVERGING (Convex) Thickest at the centre DIVERGING (Concave) Thinnest at the centre

Ray Diagrams for Lenses 21/09/2018 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

Using lenses to correct vision 21/09/2018 Short Long

What happens when light hits a material? 21/09/2018 Here are four possibilities. What are they called? Glass Wood X Mirror A rough surface Absorption Diffuse reflection Specular reflection Transmission

Investigating filters 21/09/2018 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

Red Blue Green White Yellow Cyan Magenta 21/09/2018 Red Blue Green White Yellow Cyan Magenta

Using filters 21/09/2018 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

Seeing colour revision 21/09/2018 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

A white hat would reflect all seven colours: 21/09/2018 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

Using coloured light 21/09/2018 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

In different colours of light this kit would look different: 21/09/2018 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

What colour would the following objects look like? 21/09/2018 Object Colour of light Colour object seems to be Red socks Red Blue Black Green Blue teddy Green camel Magenta book Homework