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AQA 2011 Physics Unit AQA 2011 Physics Unit 11

W Richards

This PowerPoint supports section P1.5 of the 2011 AQA Physics Unit 1 module

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An introduction to WavesAn introduction to Waves

21/04/23Some definitions…Some definitions…

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)

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““Wave behaviour”Wave behaviour”

Anything that travels as a wave demonstrates wave behaviour – in other words, it can be reflected, refracted and diffracted:

Reflection

Refraction Diffraction

21/04/23Transverse vs. longitudinal Transverse vs. longitudinal waveswaves

Transverse waves are when the displacement is at right angles to the direction of the wave (e.g. light and other electromagnetic waves)…

Longitudinal waves are when the displacement is parallel to the direction of the wave (e.g. sound waves)…

Dis

pla

cem

en

tDirection

Direction

Displacement

Where are the compressions and rarefactions?

21/04/23The Electromagnetic The Electromagnetic SpectrumSpectrum

Gamma rays

X-rays Ultra violet Visible light

Infra red Microwaves

Radio/TV

Each type of radiation shown in the electromagnetic spectrum has a different wavelength and a different frequency:

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 __ _______).

Words – black, microwaves, long, short, TV aerial, vacuum

High frequency, _____ wavelength

Low frequency, _____ (high) wavelength

γ

21/04/23The Electromagnetic The Electromagnetic SpectrumSpectrum

Type of radiation Uses Dangers

Gamma rays

X rays

Ultra violet

Visible light

Infra red

Microwaves

TV/radio

Treating cancer, sterilisation

Medical

Sun beds

Seeing things

Remote controls, heat transfer

Satellites, phones

Communications

Cell mutation

Cell mutation

Skin cancer

None (unless you look at the sun)

Sunburn

Very few

Very few

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The Wave EquationThe Wave Equation

All E-M waves obey the Wave Equation:

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

in m/s in Hz in m

V

f

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1) A water wave has a frequency of 2Hz and a wavelength of 0.3m. How fast is it moving?

2) 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?

3) 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?

4) Purple light has a wavelength of around 6x10-7m and a frequency of 5x1014Hz. What is the speed of purple light?

Some example wave equation Some example wave equation questionsquestions

0.2m

0.5m

0.6ms-1

3x108ms-

1

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How sound travels…How sound travels…As we know, sound waves are formed when something vibrates. But how does the sound reach our ears?

1) An object makes a sound by vibrating

2) The vibrations pass through air by making air molecules vibrate

3) These vibrations are picked up by the ear

Air molecules

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Transmitting informationTransmitting informationAlthough E-M radiation travels in straight lines, we can send infra-red and light signals around a curved path using an optical fibre:

Optical fibres have two main advantages: they can send more information compared to electrical cables of the same diameter and with less signal weakening.

Microwaves are used by satellites because they can pass through the Earth’s atmosphere:

Microwaves are also used in mobile phone networks.

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ReflectionReflection

Incident ray

Normal

Reflected ray

Angle of incidence

Angle of reflection

Mirror

Angle of incidence = Angle of reflection

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Evidence about the Evidence about the origins of the universe…origins of the universe…

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Source of light “Spectra

”

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If you pass the light through a gas something different is seen…

helium

Some wavelengths of light are absorbed by

the gas – an “absorption spectrum”.

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After

helium

If the light source is moving away the absorption spectra look a little different…

helium

Before

21/04/23The absorption lines have all been “shifted” towards the longer wavelength end (red end)…

After

Before

A similar effect happens with sound – this is called “The

Doppler Effect”

This is called red shift. The faster the light source moves the further its light will be “shifted”

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Red Shift simplifiedRed Shift simplified

Basically, if I walk towards you I’ll look slightly more blue. Then, if I

walk away from you, I’ll look slightly more red!!

Let’s try it…

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Light from different stars and from the edge of the universe also shows this “red-shift”. This suggests that everything in the universe is moving away from a single point.

This is the BIG BANG theory

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Red shift summaryRed shift summaryLight from other galaxies has a longer _________ than expected. This shows that these galaxies are moving ____ from us very quickly. This effect is seen to a greater extent in galaxies that are _______ away from us. This indicates that the further away the galaxy is, the ______ it is moving.

Words to use – faster, away, universe, big bang, billion, wavelength, further

This evidence seems to suggest that everything in the universe is moving away from a single point, and that this process started around 15 _____ years ago. This is the ____ ________ Theory. Further evidence of this theory is Cosmic Microwave Background Radiation (CMBR) – this radiation comes from the Big Bang and fills the _________.

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This slideshow has been made freely available on the TES Resources website.

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