Waves

Seite 1

Transverse and Longitudinal

Waves travel through a medium - particles of medium oscillate and transfer energy between each other

Particles stay in same place - only energy is transferred

Amplitude - maximum displacement of a point on wave from its undisturbed position

Wavelength - distance between same points on two adjacent waves

Frequency - number of complete waves passing a certain point per second

Measured in hertz

1Hz is 1 wave per second

From frequency, you can find period of a wave - amount of time it takes for a full cycle of wave

All waves are either transverse or longitudinal

Transverse waves

Oscillations are perpendicular to direction of energy transfer

Most waves are transverse:

All electromagnetic waves

Ripples and waves in water

Wave on a string

Longitudinal Waves

Oscillations are parallel to direction of energy transfer

Example is sound waves

Wave speed - speed at which energy is being transferred/speed wave is moving at

Wave equation applies to all waves:

Wave speed = frequency x wavelength

Seite 2

Behaviour and EM Waves

When a wave meets a boundary between 2 materials, 3 things can happen:

Wave is absorbed by second material

Wave transfers energy to material's energy stores

Often, energy is transferred to thermal energy store - leads to heating

Wave is transmitted through second material

Wave carries on travelling through new material

Often leads to refraction

Can be used in communications as well as lenses of glasses and cameras

Wave is reflected

Where incoming ray is neither absorbed or transmitted

It's instead sent back away from second material

How echoes are created

EM spectrum

EM waves are transverse waves

Transfer energy from source to an absorber

All EM waves travel at the same speed through air or vacuum

Aren't vibrations of particles, they're vibrations of electric and magnetic fields

Means they only travel through a vacuum

Travel at different speeds in different materials (can lead to refraction)

EM waves vary in wavelength from around 10^-15 to more than 10^4

Group them based on wavelength and frequency - seven basic types

Different groups merge to form a continuous spectrum

Our eyes can only detect a small part of this spectrum - visible light

Such a long range of frequencies because EM waves are generated by a variety of changes in atoms and nuclei

Also explains why atoms can absorb a range of frequencies - each one causes a different change

Because of their different properties, different EM waves are used for different purposes

Seite 3

Refraction

When a wave crosses a boundary between two materials - changes speed

If wave is travelling along normal it will change speed - not refracted

If wave hit boundary at an angle it changes direction - it's refracted

Wave bends towards the normal if it slows down

Bends away from the normal if it speeds up ]

Refraction depends on density of two materials

Usually depends on density of 2 materials

Usually higher the density of a material, slower a wave travels through it

Optical density of material is a measure of how quickly light can travel through it

Higher the optical density, slower light waves travels through it

Wavelength of a wave changes when it's refracted, but frequency stays the same

Ray diagrams

Rays are straight lines that are perpendicular to wave fronts

Show direction a wave is travelling in

Can construct a ray diagram for a refracted light ray

Wave front diagrams

Wave front is a line showing all points on a wave that are in the same position as each other after a given number of wavelengths

When a wave crosses a boundary at an angle, only part of a wave front crosses boundary first

If it's travelling into a denser material, that part travels slower than the rest of the wave front

So by the time the whole wave front crosses the boundary, faster part of wave front will have travelled further than the slower part of wave front

This difference in distance travelled by the wave front causes wave to refract

Seite 4

Radio Waves

EM waves are made up of oscillating electric and magnetic fields

Alternating currents are made up of oscillating charges

As the charges oscillate, they produce oscillating electric and magnetic fields

Frequency of waves produced will be equal to frequency of alternating current

Can produce radio waves using an alternating current in an electrical circuit

Object in which charges oscillate to create radio waves is a transmitter

When transmitted radio waves reach receiver, radio waves are absorbed

Energy carried by waves is transferred to electrons in material of receiver

Energy causes electrons to oscillate and, if receiver is part of a complete electrical circuit, it generates an alternating current

Current has same frequency as radio waves that generated it

Radio waves are EM radiation with wavelengths longer than about 10cm

Long-wave radio can be transmitted across the world

As long wavelengths diffract around curved surface of Earth

Long wave radio wavelengths can diffract around hills, tunnels etc

Makes it possible for radio signals to be received even if receiver isn't in line of sight of transmitter

Short-wave radio signals can be received at long distances from transmitter

As they're reflected from ionosphere - an electrical layer in Earth's upper atmosphere

Bluetooth uses short-wave radio waves to send data over short distances between devices without wires

Medium-wave signals can reflect from ionosphere, depending on atmospheric conditions and time of day

Radio waves used for TV and FM radio transmissions have very short wavelengths

To get reception, you must be in direct sight of transmitter - signal doesn't bend or travel far through buildings

Seite 5

EM waves and uses

Communication to and from satellites uses microwaves

Need to use microwaves which can pass easily through Earth's watery atmosphere

For satellite TV, signal from transmitters transmitted into space where it's picked up by satellite receiver dish orbiting thousands of km above Earth

Satellite transmits signal back to Earth in different direction where it's received by a satellite dish on ground

There is slight time delay between signal being sent and received because of long distance signal must travel

In communications, microwaves used need to pass through Earth's watery atmosphere

In microwave ovens, microwaves need to be absorbed by water molecules in food - so use different wavelength to those used in satellite communications

Microwaves penetrate up to few cm into food before being absorbed and transferring energy they're carrying to water molecules in food, causing water to heat up

Water molecules then transfer energy to rest of molecules in food by heating - which quickly cooks food

Given out by all objects - and hotter object, more radiation it gives out

Cameras can be used to detect infrared radiation and monitor temp

Camera detects IR radiation and turns it into an electrical signal, which is displayed on a screen as a picture

Hotter object is, brighter it appears

Absorbing IR radiation causes objects to get hotter

Food can be cooked using IR radiation - temp of food increases when it absorbs IR radiation

Electric heaters heat a room in the same way

Contain long piece of wire that heats up when current flows through it - emits lots of IR radiation

Emitted IR radiation is absorbed by objects and air in room - energy is transferred by IR waves to thermal energy stores of objects, causing temp to increase

Optical fibres are thin glass r plastic fibres that can carry data over long distances as pulses of visible light

Work because of refection - light rays are bounced back and forth until they reach end of fibre

Visible light is used in optical fibres as it's easy to refract light enough so that it remains in a narrow fibre

Light isn't easily absorbed or scattered as it travels along a fibre

Fluorescence is a property of certain chemicals - where UV radiation is absorbed and then visible light is emitted

That's why fluorescent colours look so bright - actually emit light

Generate UV radiation - absorbed and re-emitted as visible light by layer of phosphorous on inside of bulb

Energy efficient so they're good to use when light is needed for long periods

Security pens can be used to mark property

Under UV light, ink will glow but it's invisible otherwise

Can help police identify your property if it's stolen

UV radiation is produced by Sun, and exposure to it is what gives people a suntan

When it's not sunny, some people go to training salons where UV lamps are used to give an artificial suntan

Overexposure to UV radiation can be dangerous

Radiographers in hospitals take X-ray photographers of people to see if they have any broken bones

X-rays pass through flesh but not so easily through denser material like bones or metal

Amount of radiation that's absorbed/not absorbed that gives an x-ray image

Radiographers use X-rays and gamma rays to treat people with cancer

Because high doses of these rays kill all living cells - so they're carefully directed towards cancer cells, to avoid killing too many normal cells

Gamma radiation can also be used as medical tracer - where gamma-emitting source is injected into patient, and progress is followed around body

Gamma radiation is well suited to this because it can pass out through body to be detected

Both X-rays and gamma rays can be harmful to people

Radiographers wear lead aprons and stand behind lead screen or leave room to keep exposure to a minimum

Seite 6

Dangers of EM waves

When EM radiation enters living tissue - often harmless, but sometimes it creates havoc

Effects of each type of radiation are based on how much energy wave transfers

Low frequency waves - don't transfer much energy and so mostly pass through soft tissue without being absorbed

High frequency waves - transfer lots of energy and so cause lots of damage

UV radiation damages surface cells - can lead to sunburn and cause skin to age prematurely

Some serious effects are blindness and an increased risk of skin cancer

X-rays and gamma rays are types of ionising radiation

Can cause gene mutation or cell destruction and cancer

Whilst UV radiation, X-rays and gamma rays can all be harmful, they're also very useful

Before any of these types of EM radiation are used to look at whether benefits outweigh health risks

Radiation dose is a measure of risk of harm from body being exposed to radiation

Not a measure of total amount of radiation that has been absorbed

Risk depends on total amount of radiation absorbed and how harmful type of radiation is

A Sievert is big, so you'll often see doses in millisieverts

Where 1000mSV = 1 Sv

CT scan uses X-rays and computer to build picture of inside of a patient's body

Table shows radiation dose received by 2 different parts of patient's body when having CT scans

If patient has CT scan on their chest, they're four times more likely to suffer damage to their genes than if they had a head scan