P1.5 Waves

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• Created by: Fiona S
• Created on: 22-02-15 12:06

Tranverse Waves

All waves transfer energy from one place to another.

They are often used to communication, mobile phones, radios, TV(television). There are 2 types of waves:

1. Transverse

The vibrations that make the wave are at 90°/perpendicular to the direction the wave is travelling

Example
All
waves in the electromagnetic spectrum. Transverse waves can travel through a vacuum.

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Longitudinal Waves

The vibrations are in the same direction/parallel to the direction the wave is travelling.

Example

Sound waves and spectrum waves, seismic waves.

Longitudinal waves cannot travel through a vacuum.

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Electromagnetic Spectrum

Electromagnetic waves are electro and magnetic disturbances which transfer energy from 1 place to another.

(longest wavelength, least energy) Radiowaves, Microwaves, Infrared, Visible-Light, UV, X-Ray, Gamma (shortest wavelength, most energy)

Properties of all electromagnetic waves

1.They all transfer energy from one place to another
2.They are all transverse waves
3.They can all be reflected, refracted and diffracted
4.They can all travel through a vacuum(space)
5.They all travel at 300,000,000 m/s in a vacuum
6.Wave Speed = frequency x wavelength
7.The shorter the wave length (the higher the frequency), the more dangerous they are.

c = f x λ
c = 300,000,000 m/s                (3 x 10^8 m/s)

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Electromagnetic Spectrum

Gamma Rays
Typical wavelength: 10^-12m (a million-millionth of a meter)
Sources: Radioactive substances like uranium, very dangerous
Detector: Geiger-Muller Tube

X-Rays
Typical Wavelength:
10^-10m (a ten thousand-millionth of a meter)
Sources: X-Ray tubes, very dangerous
Detector: photographic film

U-V
Typical Wavelength:
10^-8m (a hundered-millionth of a meter)
Sources: Very hot objects, sun, sparks, mercury lamps, dangerous
Detector: photographic film, causes sun tan, make fluoresent substances glow

Visible Light
Typical Wavelength:
5 x 10^-7m (a two-millionth of a meter)
Sources: Hot objects, sun, fluoresent substances, laser, LEDS
Dectector: Eyes, Photographic film, an LDR

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Electromagnetic Spectrum

Infrared
Typical Wavelength:
10^-5m (a hundered-thousandth of a meter)
Sources: Irons, fire, grills, toasters, warmth of hot objects
Detectors: Skin, a blackened thermometer, a thermistor

Microwaves
Typical Wavelength:
10^-3m to 10^-2m
Sources: Microwave ovens, satellites and mobile phones
Detectors: a mobile phone

Typical Wavelength:
10^-1m to 10^3m
Detectors: Aerial with a TV set or a radio set

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Uses:
Dangers:
No danger
How to reduce the danger:
No need to reduce

Microwaves
Uses:
To heat up objects e.g. food, detecting echoes from objects, satellites communication
Dangers: Heating water in tissue can cause 'burning' (can be absorbed by the water in the cells)
How to reduce the danger: Close door to microwave, don't let microwaves come into contact with skin. Use hands free phone, earpiece

Infrared
Uses:
Firefighters use infrared viewer to look for unconscious people in smokey buildings, look for survivors from earthquake
Dangers: Causes burning of tissue (absorbed by skin, cause burn)
How to reduce the danger: Reflexs stop burns

Visible Light
Uses:
To see things, light reflects into your eyes so you see colour, optical fibres
Dangers: Very bright, sensitive, light can damage eyes activites sensitive cells in retina
How to reduce danger:
Sunglasses

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Ultraviolet
Uses:
Helps produce Vitamin D in our skin
Dangers:
Can cause skin cancer and are bad for our eyes, causes tanning
How to reduce the danger:
Suncream, sunglasses

X-Ray
Uses:
Doctors and dentists use X-Rays to check bones and teeth
Dangers: Large amounts can cause cancer in high doses
How to reduce the danger: Only use when needed/necessary

Gamma Ray
Uses:
To sterilise food and used by doctors to get important medical information
Dangers: Its source is a radioactive substance, kills living cells in high doses, causes cancerous cells in lower doses, kills cancer cells
How to reduce the danger: Used very carefully, thick lead

Radiowaves cannot travel through the ionosphere

Microwaves can travel through the ionosphere

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Measuring Waves

Wavelength(λ) > Metres - the distance from a point on one wave to the next corresponding wave
Amplitude(A) > Metres - the maximum displacement from equilibrium position (height of the wave)
Time Period(T) > Seconds - the time taken for one complete cycle of the wave, a wavelength
Frequency(F) > Hertz (Hz) - the number of waves produced per second

f = 1/T        t = 1/f

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Measuring Waves continued...

The Amplitude of the wave is the height of the wave crest or the depth of the wave trough from the middle -  the bigger the amplitude of the waves, the more energy the waves carry.

The Wavelength of the waves is the distance of one wave crest to the next crest.

We need to measure waves if we want to find out how much energy or information they carry.

Wave Speed - straight waves are called plane waves. The waves all move at the same speed and keep the same distance apart. The speed of the waves is the distance travelled by a wave crest or a wave trough every second.

v = f x λ             f = v/λ               λ = v/f

v = wave speed (m/s)
f = frequency (Hz)
λ = wavelength (m)

1kHz = 1000Hz

All electromagnetic waves travel at the same speed - 300,000,000. All electromagnetic waves can travel through a vacuum and if it needs a material to travel through it's called a mechanical wave.

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Reflection

• Angle of incidence (i°) is the angle between the incident ray and the normal
• The law of reflection is that the angle of incidence is always equal to the angle of reflection
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Images formed in a plane mirror

A plane mirror is a straight mirror i.e. not curved

Properties of an image in a plane mirror

• Virtual
• Upright
• Laterally Inverted (back to front)
• The same distance behind the mirror as the object infront of the mirror
• The same size as the object
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Images formed in a plane mirror continued...

1. Draw the mirror and then draw the point where the object is
2. Draw 2 Lines from the object to the mirror and draw the normals and measure their lines of incidence from the normal
3. Measure the same angle on the other side of each of the normals (angle of reflection)
4. Mark these using complete lines, use dotted lines to mark virtual rays
5. We draw these by measuring the same angle from the normal on the other side of the mirror
6. Don't forget to draw arrows on the real light rays

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Refraction

Refraction is a property of all forms of waves. When waves travel between different density, the speed of the wave changes resulting in a change in direction.

When a light ray travels from air to glass, it refracts towards from the normal.
When a light ray travels from glass to air, it refracts away from the normal
When a light ray travels from air to glass, the angle of refraction is less than the angle of incidence
When a light ray travels from glass to air, the angle of refraction is bigger than the angle of incidence

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Refraction continued...

Refraction is the bending of a wave as it enters a more dense material and is caused by a change in speed of a wave.

As it enters more dense medium, the wave slows and bends towards the normal. As it enters a less dense medium, the wave speeds up and bends away from the normal. If the wave is along the normal there is no change in direction and the wave continues straight although the speed still changes.

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Diffraction

Diffraction is a property of all types of waves. Diffraction is the spreading out of waves as they pass through a gap or around an obsticle. When the gap width is equal to the wavelength of the waves maximum diffraction occurs.

Examples of diffraction
1. Telescopes - made having a large diameter lens/mirror so that minimal diffraction occurs giving clear and detailed images
2.Ultrasound - design of an ultrasonic scanner. The ultrasonic waves spread out from a hand held transmitter, which has been carefully designed so the waves don't spread out too much/not enough.
3.Recieving Radio(TV) Signals - if there are hills between the transmitter mast and the TV reciever (antena), the reciever may not recieve the signal

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Sound Waves

Sound waves are caused by vibrations. An object vibrating in air causes layers of air to vibrate. When the sound waves reach your ears, your ear drum vibrates in and out and we hear noise.

Sound waves are longitudinal because the particles in the wave vibrate in a direction parallel to the direction of energy transfer.

Frequency

Frequency is the number of waves produces each second. Unit Hz. Young people can hear frequencies from 20Hz up to 20,000Hz. As we get older out ability to hear the highest frequency sounds decreases.

Sound below human hearing( <20Hz) -> infrared   e.g. Elephants and Whales

Sound above human hearing(>20,000Hz) -> ultrasound   e.g. Dogs, Bats and Dolphins

Uses of ultrasound: viewing a foetus in a womb - scan
cleaning watched and jewelery

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Sound in a vacuum

Sound is a longitudinal wave as it need particles to pass on the vibrations, which make up the sound wave. This means that sound can't travel in a vacuum.

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Dispertion

Dispertion is refraction by a prism.

Each colour of light is refracted slightly differently. This is because the speed of light in glass depends on the wavelength. Violet light has a shorter wavelength than red light. This means it travels slower in glass. So it is refracted more.

Below violet light is ultraviolet which you can detect with a ultraviolet detector and above red light is infrared radiation which you can detect by temperature.

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The Doppler Effect

This was discovered by Christian Doppler in 1842, the Doppler effect is a change in the observed wavelength and frequency of any wave due to movement of the wave source.
When a moving object emits waves they can become streched or compressed.
Remember white light contains all the colours of the spectrum.

Red light has the longest wavelength                        Blue light has the shortest wavelength

When an object is travelling towards us, the waves are compressed, this means they have a shorter wavelength, we say they have been blue-shifted.
When an object is travelling away from us, the waves are spread out, this means they have a long wavelength, we say they are red-shifted.

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The Big Bang Theory

The theory of how the universe was created. All matter from the universe was pulled together and created what we see today and is expanding.

Evidence 1

An object moving away from the observer has a longer wavelength. This is due to the Doppler effect.
The spectrum from distant galaxies experience red shift. This is because red light has the longest wavelength.
This shows that the universe is expanding.
A further away galaxy has a larger red shift this means that this galaxy is moving away at a faster speed.

Evidence 2

Cosmic Microwave Background Radiation (CMBR) - Detected heat left over from the Big Bang. Heat comes from explosions.

The heat is thought to be from the big bang and it is used (CMBR) to detect heat from the far off galaxies.

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The Big Bang Theory

Theory 1: The Big Crunch

Gravity will stop the Universe from expanding. It will start contracting, everything would be destroyed. 'Another Big Bang'? All particles would be totally different.

Theory 2: The Big Chill

The Universe will keep expanding forever. Everything will be too far apart. No source of light and heat - all stars eventually die.

The big bang theory states that the universe began 14 billion years ago from a small point in a huge explosion and has been expanding ever since.

Evidence for the big bang theory

1.Red Shift - all galaxies are moving apart so must have previously been closer

2. CMBR - Relic of the gamma radiation released in the big bang theory

3. Abundence of elements - BBT predicts 90% Hydrogen and 10% Helium, later discovered to be correct.

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