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EM Waves (Dangers of EM waves (The effects of each type of radiation…
EM Waves
Dangers of EM waves
The effects of each type of radiation depend on how much energy the wave transfers. (Remember the higher the frequency the higher the energy)
Low frequency waves like radio waves don't transfer much energy and pass through soft tissue without being absorbed
High frequency waves like UV, X-Rays and Gamma rays, transfer lots of energy and cause lots of damage
UV damages surface cells which can lead to sunburn, cause skin to age prematurely, blindness, and increased risk of skin cancer
X-rays and gamma rays are types of ionising radiation. They have enough energy to knock electrons out of atoms, which causes gene mutation and possibly cancer
Before using any type of ionising radiation, people must look at whether the benefits outweigh the risk (it is better to scan for broken bones in an accident and risk developing cancer later than not checking)
Radiation dose, measured in sieverts (Sv), is a measure of the risk of harm from the body being exposed; it is not a measure of the amount of radiation absorbed. The risk depends on the total amount absorbed and the type of radiation. Radiation dose can be calced for all types, not just UV X-rays and gamma
A sievert is a lot, so doses are usually in millisieverts (mSv)
Risk can be different for different parts of the body. A CT scan on the chest is four times more likely to suffer from gene damage than a head scan.
Radio Waves
EM waves are made up of oscillating electric and magnetic fields. Radio waves can be produced by using an AC current in an electrical circuit. The object in which charges (electrons) oscillate is called a transmitter
Receivers absorb the waves, and the energy is transferred to the electrons in the material of the receiver. This cause the electrons to oscillate, and produce an AC current of the same frequency as the one supplied to the transmitter
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Long-wave radio waves (λ 1-10km) can actually diffract (bend) around the earth and can be received from the other side of the world, which means the receiver doesn't have to be in the line of sight of the transmitter
Short-wave (λ 10-100m), can also travel far, but they reflect off the ionosphere, as can medium-wave signals, depending on atmospheric conditions and time of day.
Bluetooth uses short-wave radio to send data over very short distances, like bluetooth headsets
Radio waves in TV and FM radio have very short wavelengths and must be in direct sight of the transmitter, the signal doesn't bend or travel far through buildings
Infrared radiation (IR)
Given out by all objects, the greater the heat the more given out
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Can be detected by cameras, the camera detects the radiation and turns it into an electrical signal which is displayed as a picture on screen. The hotter the object the brighter
Absorbing IR radiation causes objects to get hotter, food is cooked with IR radiation. Energy is transferred by by IR waves to the thermal energy stores of objects
Electric heaters have a long piece of wire that heats up and emits IR radiation (and some visible light) when a current flows through it.
They're also used for transmitting signals from TV remotes, they are good for this because they can be easily produced and detected electronically
Visible light
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Used in optical fibres; thin glass or plastic fibres that can carry data over long distances as pulses of visible light. Light rays are reflected back and forth inside the fibre and not absorbed or scattered.
The visible light spectrum covers violets 400nm - 700nm reds (UV is shorter than violet and IR is longer than red)
Each colour has its own wavelengths and frequencies, and different mixtures of these create different shades. All of these put together create white light, and the absence is black.
Colour and Transparency
Different objects absorb, transmit and reflect different wavelengths of light in different ways
Opaque objects do not transmit light. They absorb some wavelengths and reflect others. The colour of an opaque object depends on which wavelength of light are most strongly reflected. e.g. when white light hit apple, apple absorb all colour but red and red go into eye
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Opaque objects that aren't a primary colour can be reflecting either the wavelengths of light of its colour OR reflecting multiple primary colours to make that colour e.g. yellow banana either reflect yellow light OR reflect red and green light
White objects reflect all of the wavelengths of visible light EQUALLY, and black objects absorb all of it
Transparent and translucent objects transmit some of the light that hits it. Some wavelengths can be absorbed or reflected by these objects, and its colour is related to wavelengths that are transmitted and reflected
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ROYGBIV. The brain has cones for RGB, so they are the three primary colours
UV radiation
Fluorescence is a property of some chemicals, where UV is absorbed and visible light is emitted, which is why they look so bright.
Fluorescent lights generate UV radiation, which is reabsorbed and re-emitted as visible light. They are energy efficient, and so good when light is needed for a long time
Security pens can be used to mark property, it will be invisible unless UV light is shone on it, which can help police identify property. This is also how those invisible ink pens work
UV radiation is produced by the Sun, and exposure to it gives suntan/burn
Tanning salons produce harmful amounts of UV radiation for tanning, unlike fluorescent lights, which emit very little UV
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Microwaves
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Used in satellites, including satellite TVs and phones.
For TV, the signal from a transmitter is transmitted into space through the Earth's watery atmosphere, where it's picked up by the satellite receiver dish, orbiting the earth. From there, the satellite transmits the signal back to Earth in a different direction where it's received by a satellite dish on the ground. There is a time delay due to distance.
Also used in microwave ovens, but the λ is different for those. The microwaves are absorbed by water molecules in food. The waves only penetrate a few cm into the food before being absorbed and transferring energy and causing food to heat up. Water molecules transfer by heating, and the food is cooked quickly
X-Rays
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X-rays pass easily through flesh, but not easily through denser material like bones or metal. The amount of radiation absorbed (or not) gives the image. The brighter bits are where fewer X-rays get through. X-ray photos are negative, the plate starts off all white
Both X-rays and gamma rays are harmful to people so radiographers wear lead aprons and stand behind a lead screen so they don't get cancer
Gamma Rays
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Used in conjunction with X-rays to treat people with cancer (radiotherapy). High doses of these rays kill all living cells, so they are aimed towards cancer cells only.
Also used for medical tracing; a gamma-emitting source is injected into the patient and it is followed around the body. Gamma is good for this because it can pass out through the body to be detected.
All EM waves travel at the same speed through air or a vacuum (~30,00,00,000 m/s)
Radio Waves (1m-10km) > Micro Waves (1cm) > Infrared (10μm) > Visible Light (100nm) > UV (10nm) > X-Rays (0.1 nm)> Gamma Rays (0.0001 nm)
Increasing frequency, energy and decreasing wavelength
Generated by a variety of changes to atoms and their nuclei. e.g. changes in the nuclei cause gamma rays
EM waves are made up of oscillating electric and magnetic fields.The object in which charges (electrons) oscillate is called a transmitter. Something that receives EM waves is a receiver, and the energy from the wave is transferred to the electrons in the material of the receiver