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Waves (Wave behaviour and EM waves (When wave meets a boundary between…
Waves
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Refraction
Refraction- waves changing at a boundary. When wave crosses boundary between 2 materials, it changes speed. If wave is travelling along normal, it will change speed but not refracted. If wave hits boundary at an angle, it changes direction- refracted.
Wave bends towards normal if it slows down. Bends away from normal if it speeds up. Amount of refraction depends on speed or slowing down of wave- depends on density of 2 materials; the higher the density, the slower a wave travels through it.
Optical density of material- how quickly light can travel through it- the higher the optical density, the slower light waves travel through it. Wavelength of wave changes when refracted but frequency stays the same.
Rays- straight lines that are perpendicular to wave fronts. Show the direction a wave is travelling in.
Angle of incidence- angle between incident ray and normal.
Angle of refraction- angle between refracted ray and normal.
If second material is optically denser than first, refracted ray bends towards normal and angle of refraction is smaller than angle of incidence.
If second material is less optically denser, angle of refraction is larger than angle of incidence.
Wave front- line showing all points on a wave that are in same position as each other after given number of wavelengths.
When wave crosses boundary at an angle, only part of wave front crosses the boundary at first. If it travels into a denser material, that part travels slower than rest of wave front. So, by time the whole wave will have crossed the boundary but the faster part of the wave will have travelled further than slower wave front. This difference in distant travelled (caused by difference in speed) by wave front causes wave to refract.
Radio waves
EM waves made of oscillating electric and magnetic fields. Alternating currents are made up of oscillating charges. As charges oscillate, produce oscillating electric and magnetic fields, i.e. EM waves.
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Radio waves produced using an alternating current in electric circuit. The object in which charges (electrons) oscillate to create radio waves is transmitter. When transmitted radio waves reach receiver, radio waves are absorbed.
Energy carried by the waves is transferred to electrons in material of receiver. Energy causes electrons to oscillate and if receiver is part of complete electrical circuit, it generates alternating current- has same frequency as radio waves that generated it.
Radio waves are EM radiation with wavelengths longer than about 10 cm. Long wave radio (wavelength 1-10 km) can be transmitted from London and received half-way around the world- long wavelengths diffract around curved surface of the earth.
Long wave radio wavelengths also diffract around hills, into tunnels etc. Makes it possible to for radio signals to be received even if receiver isn't in line of sight of transmitter.
Short wave radio signals (10-100 m) can be received at long distances from transmitter because they are reflected from ionosphere- electrically charged layer in Earth's upper atmosphere.
Medium wave signals can also reflect from ionosphere depending on atmospheric conditions and time of day.
Bluetooth uses short-wave radio waves to send data over short distances between devices without wires (wireless headsets so use phone when driving car). Radio waves used for TV and EM 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.
EM waves and uses
Communications to and from satellites uses microwaves but uses microwaves which can pass easily through Earth's watery atmosphere.
For satellite TV, signal from a transmitter is transmitted into space where it's picked up by satellite receiver dish orbiting thousands of kilometers above Earth. Satellite transmits signal back to Earth in a different direction where received by a satellite dish on the ground. There is slight time delay between signal being sent and received because of long distance the signal has to travel.
In communications, microwaves used need to pass through Earth's watery atmosphere. In microwave ovens, microwaves need to be absorbed by watery molecules in food-- use different wavelengths to those used in satellite communications.
Microwaves penetrate up to a few cm into food before being absorbed ad transferring energy to water molecules; causing water to heat up- then transfers this energy to rest of molecules in food by heating- quickly cooks food.
Infrared radiation (IR) is given out by all objects; the hotter the object, the more IR radiation it gives out. Infrared cameras can be used to detect IR radiation and monitor temp. Camera detects IR radiation and turns it into an electrical signal, which displayed on screen as picture. The hotter an object, the brighter it appears e.g. thermal energy in a house detected b infrared cameras.
Absorbing IR radiation causes object to get hotter. Food can be cooked doing this- temperature of food increases when absorbs IR radiation e.g. from toasters heating element.
Electric heaters heat a room in the same way. they contain a long piece of wire that heats up when current flows through it. This wire them emits lots of IR ( and little visible light- wire glows). The emitted IR is absorbed by objects and air in room- energy is transferred by IR waves to thermal energy stores of objects- causing increased temp.
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More uses of EM waves
Optical fibres- thin glass or plastic fibres that carry data (telephones, computers) over long distances as pulses of visible light. Work because of refraction. Light rays are bounced back and forth until they reach end of fibre. Visible light used in optical fibres because easy to refract light enough so that it remains narrow fibre. Light is also not easily absorbed or scattered as it travels along fibre.
Fluorescence- property of certain chemicals where UV radiation is absorbed and then visible light is emitted-- look bright.
Fluorescence lights generate UV radiation which is absorbed and re-emitted as visible by layer of phosphorous on inside of bulb- energy efficient so good for long periods.
Security pens can mark property with name (laptops). Under UV light, ink will glow but invisible otherwise- can help identify stolen property.
UV radiation- produced by sun and exposure gives suntan. When not sunny, tanning salons; UV lamps give artificial suntan- overexposure can be dangerous. (fluorescence light emit little UV-safe).
Radiographers in hospitals take x-rays photographs of people to see broken bones-- pass easily through flesh but not through denser materials like bone or metal. So amount of radiation that's absorbed gives x-ray image.
Use x-rays and gamma rays to treat cancer- radiotherapy. Because high doses of these rays kill all living cells- carefully directed towards cancer cells to avoid killing too much healthy cells.
Gamma radiation can be used as medical tracer- gamma emitting source injected into patient and progress followed around body. Gamma radiation is well suited because can pass out through body to be detected. X-rays and gamma rays can be harmful to people so radiographers wear lead aprons and stand behind lead screen or leave room to keep exposure at a minimum.
Dangers of EM waves
When EM radiation enters living tissue; often harmless but sometimes creates havoc. Effects based on how much energy waves transfer.
Low frequency waves (UV, X-rays and gamma rays) transfer a lot of energy so cause damage.
UV- damages surface cells; sunburn and cause skin to age prematurely as well as blindness, increased risk of skin cancer.
X-ray and gamma- gene mutation or cell destruction or cancer.
EM radiation harmful but also useful but before use, must see if benefits outweigh health risks. Radiation risk (sieverts) is a measure of risk of harm from body being exposed to radiation-- not a measure of total amount of radiation absorbed. Risk depends on total amount f radiation absorbed and how harmful type of radiation is- 1000 msv= 1 sv.
CT scan uses X-rays and computers to build picture of inside of patients body.
Head radiation dose- 2.0 sv
Chest radiation dose- 8.0 sv
If a patient has CT scan on chest, 4x more likely to suffer damage to genes (added risk is also 4x higher) than head scan.