Waves

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Wave properties

 When waves pass through a medium, the particles oscillate and transfer energy however the overall particles stay in the same place.

  • Amplitude - the maximum displacement of a point on the wave from its undisturebed point.
  • Wavelength - distance between the same point of two adjacent waves.
  • Frequency - the number of complete waves passing a point per second, messured in Hz.
  • Period -the amount of time it takes for a full cycle of a wave.   
  • Wave speed - speed at at wgoch energy is being transferred.

Wave speed = frequency (Hz) x wavelength (m)                             

Period (s) = 1 / Frequency (Hz)

Transverse waves - the oscillates are perpendicular to the direction of energy transfer. This includes all electromagnetic waves, ripples and waves in water, a wave on a string.

Longitudinal waves - The oscillations are parallel to the direction of energy transfer. For example soundwaves in air like ultrasound.

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Experiments with waves

To measure the speed of sound.

Attatch a signal generator to a speaker and generate a speifc frequency. Attach two mirophones and an oscillope.

  • Set up the osillator so the deteced waves at each micophone are shown as seperate waves.
  • Start with both microwaves next t the speaker then slowly one away until the two waves are aligned on the display but have moved exactly one wavelength apart.
  • Measure the distance between the microphones to find one wavelength and use the formular to find the wave speed,
  • The speed of sound is 330m/s.

To measure the speed of ripples.

  • Use a signal generator attatched to the dipper of a ripple tank to ceate waves with a set frequency.
  • Use strobe light to see the wave sests on a screen below the tank and increase the frequency until the wave pattern appears to "freeze". This happens when the frequency of the strobe light is equal to the frequency of the waves.
  • The distance between each shadow line is equal to one wavelength, therefore measure the dstance between 10 and divide by 10 to find the aveage.
  • Use wave speed = frequency x wavelengh
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...continued

Waves on strings

Attatch a signal generator, vibration transducer, piece of string, pulley attachted to a bench and masses.

  • Turn on the signal generator and vibration trnsducer so that the sting starts to vibrate.
  • Adjust the frequency on the sinal generator until theres a clear wave on the string, this will depend on the length between the pulley and induer and the masses.
  • Measue the lengths of a few half-wavelengths then divide to get the mean-half wavelength. Then double to get the full wavelength.
  • Find the speed using the equation wave speed = frequency x wave length.
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Wave Behaviours

When a wave meets a boundary between two material, theres three things that can happen:

  • The wave is absorbed by the second material and transfers energy to its stores (often into the thermanl energy store causing a temperature change).
  • The wave is transmitted through the second material and carries travelling through. This often leads to refraction  and is used in communication and the lenses of glasses and cameeras.
  • The wave is refelected and "sent back" away from the second material, for example echos.

What happens depends on the wavelength of the wave and the properties of the materials involved.

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Refraction

  • When a waves crosses the boundary of two materials it changes speed and is refracted if not travelling along the normal.
  • In a denser material, the wave bends towards the normal and slows down.
  • How much its refracted by depends on density of the material and how much it has been speeded up by or slown down.
  • The optical density of a material is a measure of how quickly light can travel through it - the higher the optical density the slower the light travels through it.
  • The wavelength of a wave changes when its refracted but the frequency stays the same.
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Diagrams

Ray diagrams

Rays are straight lines that are perpendicular to wave frontsand show the direction a wave is travelling in. If the second material is optically denser, the refracted ray will bend towards the normal and the angleof refraction is smaller than the angle of incidence. However if it is less dense the angle will be smaller.

Wave front Diagrams

A wave front is a line showing all of the points of a wave that are in the same position as each other after a given number of wavelengths. When a wave crosses an angle at a boundary only part of the wave front crosses the boundary at first. For example, if it's entering a denser material the first part travels slower than the rest of the wave front. Therfore, when all of the wave has crossed the boundary, the faster part of the wave will have travelled further, causing the wave to bend/refract.

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EM wave spectrum

  • Electromagnetic waves are transverse waves and transfer energy from a source to an absorber. For example, a camp fire transfers infrared radiation to its surroundings, it is then absorbed by objects, transerfing energy to their thermal energy store.
  • All EM waves travel at the same speed through air or a vacuum, however they travel at differet speeds through different materials. They can travel through a vacuum as they're vibrations of electric and magnetic fields.
  • EM waves vary in wavelength from around 10^-15m to more than 10^4m. They are grouped based on their wavelength and frequency.
  • There is such a large range of frequencies as EM waves are generated by a variety of changes and their nuclei e.g. changes in the nucleus of an atom create gamma rays. This also explains why atoms can absorb a range of frequencies.
  • Because of their different properties, different EM waves are used for different purposes.
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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 e.g. electromagnetic waves.
  • The frequency of the waves produced will be equal to the frequency of the alternating current.
  • Using an alternating current in an electric curcuit, radio waves can be produced when electrons in a transmitter oscillate.
  • When transmitted radio waves each a reciever, they are absorbed and the energy is transferred to the electrons of the new material.
  • This energy causes the electrons to oscillate and, if the reciever is part of a full circuit, it generates and alternating current with the same frequency as the radio waves.

Communication

Radio waves have wavelengths longer than about 10cm. Long-wave radio (wavelengths of 1-10 km) can be transmitted around the world. Short wave radio signals (10m -100m) are reflected from the inosphere - an electrically charged layer in the earth's upper atmosphere - and so can be recieved at long distances. Short wave radio waves can be sent over short distances, for example bluetooth. Medium wave signals can also be reflected from the ionosphere dependent of the weather conditions and time of day. The waves used for TV and FM radio are short wavelengths and do not bend or travel through buildings.

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Microwaves

Microwaves have a shorter wavelength than radio waves.

Communications to and from satellites use microwaves which can pass easily through the earth's watery atmoshere. For a satellite TV, the signal from a transmitter is transmitted into space where it is picked up by the satalite receiver dish orbitting thousands of kilometers above the earth. The satellite then transmits the signal back to the earth where it is picked up by a satellie dish on the ground.

In microwave ovens, the microwaves need to be absorbed by water molecules in food and so require different wavelengths to those in satelitte communication. The microwaves penetrate up to a few centimetres into the food before being absorbed, transferring the energy into the water molecules in the food, causing it it heat up.

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Infrared Radiation

Infrared raditation is given out by all objects. The hotter an object is, the more it gives off.

  • Infrared cameras can be used to detect infrared radiation and monitor temperature, turning the radiation into an electric signal on a screen, displayed a picture. The hotter an object is, the brighter it appears.
  • Absorbing infrared radiation causes objects to get hotter. Therfore, it can be used to cook food (e.g in a toaster) or heat up a room (e.g. using an elctric toaster).
  • Electric heaters contain a long piece of wire that heats up when a current passes through it. This wire then emits lots of infrared radiation (and a little visible light). The energy is transferred into the thermal energy stores of other objects, causing their temperature to increase.
  • Optical fibres in communications systems often use infrared radiation instead of visible light as it is absorbed less by the glass.
  • Removet control handsets also use infrared radiation to transmit signals.
  • Infrared scanners are used in medicine to detect which parts of the body are emmiting heat and need to be treated.

Infrared radiation can be dangerous as it is absorbed by the skin and can damage, burn or kill skin cells as it causes them to heat up.

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Ultraviolet Radiation

Fluorescence is a property of certain chemicals, where ultra-violet radiation is absorbed and then visible light is emitted.

  • Fluorescent lights generate UV radiation, which is absorbed and re-emitted as visible light by a layer of phosphorus on the inside of the bulb. They are energy-efficient so very useful when needed for long periods.
  • Security pens can be used to mark property and then can be detected under a UV light as the "invisible" ink will glow. This can help the police identify stolen goods.
  • Ultraviolet light is produced by the sun and when exposed to it, gives people a suntan. However, many use tanning salons where UV lamps are used to give them an artificial tan. However, over exposure to UV radiation can be dangerous. It can cause skin cancer or the skin to age prematurely.

Ultraviolet light is harmful to the human eye and can cause blindness. This is because their wavelengths are smaller than visible light  and so carry more energy.

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Visible light

Visible light is the only EM radiation visible to the human eye.

Optical fibres are thin glass of plastic fibres that can carry data very long disatances as pulses of visible light. Using reflection, the light rays are bounced back and forth until they reach the end of the fibre. 

This can carry more information than radiowaves as it has a shorter wavelength and so can cary more pulse of wave. They are also more secure as the signals stay in the fibre.

Visible light is used in optical fibres because it is easy it is easy to refract light enough so that it remains in a narrow fibre. Light is also not easily absorbed or scattered as it travels along a fibre.

Light from an ordianry lamp or from the sun is called white light and contains all the colours of the visible spectrum, it is used in two ways:

  • In a film camera, the light is focused by the camera lens on a light-sensitive film which is then develpoed to see the image.
  • In a digital camera the light is focused by the lens to a sensor made up of thousands of ligh-sensitive cells called pixels. Each pixel gives a dot of the image, which can be seen on the screen.
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X-rays and Gamma rays

Radiographers in hospitals take X-ray "photographs" of people to detect broken bones. This is because X-rays can easily pass through flesh but not through denser materials like bone and metal.

X-rays are produced when when electrons or other particles moving at high speeds are stopped

It is the amout of radiation abosorbed / not absorbed which gives you the image. The brighter bit of the image is where few rays get through. This is called a negative image as the plate starts of white and turns darker.

Radiographers also use X-rays and gamma rays to treat people of cancer (radiotherapy). This is becasue high doses of these rays kill all living cells. This can be directed towards the cancer cells.

Gamma rays are produced by radioactive substances when unstable nuclei release energy. They have a shorter wavelength and so can penetrate more.

Gamma radiation can also be used as a medical tracer, where a gamma-emitting source is injected into the patient and its progress followed around the body. This can be detected as it easily passes out through the body. Gamma rays are also used to kill disease-carying organisms in food and prevent food spoilage.

Both of these rays can be harmful to people so radiographers wear lead aprons and stand behind a lead screen to keep their expose to them at a minimum.

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Dangers

When EM radiation enters tissue it is often harmless, however depending on the type or amount, it can cause damage. Low frequency waves, like radio waves, dont transfer much energy and so pass through most soft tissue without eing absorbed. However, high frequency UV like X-rays and gamma rays (which are types of ionising radiation) transfer alot of energy which and can cause gene mutations, cell destruction or cancer.

The dangers of these radiation waves often do not outweigh the possitives, and therfore they are used. For example, it is much more important to find injuries using an X-ray than the potential health risks it carries.

Radiation dose (measures in sieverts) is a measue of the risk of harm from the body being exposed to radiation, depending on the total amount absorbed, the amount of time the body is exposed to it and and how harmful that type of radiation is (e.g. alpha inside the body is much more harmful than gamma). This can be different for different parts of the body. High dose can kill living cells whilst a low dose can cause gene mutation.

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Practicals

Emission.

A leslie cube is a hollow, watertight metal cube usually made of alluminium whose four fifferent vertical faces have different faces.

  • Place an empty Leslie cube on a heat-proop mat.
  • Boil water in a kettle and fill the Leslie cube with boiling water and wait for it to heat up.
  • Hold a thermometer to each surface (they should all be the same temperature)
  • Hold an infrared detector a set distance away from one of the sides and record the amount of IR radiation it detects.
  • Repeat for each vertical face.
  • You should find that matt shiny surfaces emit more than white shiny ones.

Absorption.

  • Set up two identical metal plates with a ball bearing stuck the outside with wax on either side of a bunson burner, one with a black, matt inside, the other with a silver inside.
  • The ball bearing on the black plate will plate will fall first as the black surface absorbs more infrared radiation - transferring more radiation to the energy of the thermal energy store of the wax. This means that the wax on the black plate melts before the wax on the silver plate.
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