Waves & Radiation Summary Notes
Section 1 - Sound Sound is caused by vibrations. Sound can travel through solids, liquids and gases, but not a vacuum
Ultrasound Ultrasound is sound with a frequency above the range of human hearing. (above 20 000 Hz) The Ultrasound Scanner An ultrasound scanner can be used to examine the inside of a patient (e.g. an unborn baby inside the mothers womb). Ultrasonic waves are directed into the patient and reflect off objects inside the patient. It takes different times for the sound waves to return from different depths and so an image of the patient can be built up using a computer. A layer jelly is placed on the skin to prevent sound waves reflecting off the skin.
Noise Levels Noise levels are measured in decibels , dB. Regular exposure to noise levels above 90 dB (e.g. pneumatic drills or heavy traffic) can cause damage to hearing. Typical noise levels :
0 dB 60 dB 95 dB 100 dB 130 dB 140 dB
Threshold of hearing Normal conversation Heavy lorry Pneumatic drill Jet engine Pain threshold
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Waves & Radiation Summary Notes
Section 2 - Radio Waves & Microwaves Radio Waves, Television Waves and Microwaves Mobile telephones, radio and television are all examples of long distance communication that do not require any cables between transmitter and receiver. All these methods of communication use waves to carry the signals. Mobile phones use microwaves to carry the signals, radio uses radio waves and television uses television waves. Microwave, radio and television waves all travel at very high speed (300,000,000 m/s in air). Different radio transmitters can be identified by their different frequency or wavelength values (e.g. Radio 1 has a frequency of 99.5 MHz and Radio 4 LW has a wavelength of 1500 m)
Speed, Distance and Time
The Wave Equation
(radio, television and microwaves)
(radio, television and microwaves)
m speed =
distance time
speed = frequency x wavelength
d t
m
m/s
s
m/s
v
Hz
d=vt v = dt t = dv
v=fλ f = vλ
v f
λ
λ = vf
Curved Dishes - Receivers
Curved Dishes - Transmitters
Curved reflectors are used to increase the strength of a received signal from a satellite or other source. The curved shape of the reflector collects the signal over a large area and brings it to a focus. The detector is placed at the focus so that it receives a strong signal
Curved reflectors are also used on certain transmitters to transmit a strong, parallel beam of signal. In a dish transmitter the signal source is placed at the focus and the curved shape of the reflector produces a parallel beam of signal.
parallel beam of signal
signals from a distant source signal source placed at focus
detector placed at focus
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Waves & Radiation Summary Notes Diffraction Diffraction is the bending of light around obstacles. All waves diffract to some extent, but longer wavelengths diffract more than shorter wavelengths.
long wavelength lots of diffraction
Radio waves have a longer wavelength than television waves and therefore can bend round obstacles such as hills, buildings or trees more easily. Therefore radio reception is better than television reception in such areas
short wavelength little diffraction
Satellites The time taken for a satellite to complete one orbit of the Earth depends on it’s height above the Earth; the higher the orbit of the satellite the longer it will take to orbit. Geostationary satellites take 24 hours to orbit the Earth. This is the same time that Earth takes to complete one rotation and so the satellite always remains above the same point on the Earth’s surface. Ground stations send signals to the satellite using a curved dish transmitter to transmit a strong signal. At the satellite the signal is collected by a curved dish receiver, then amplified and finally retransmitted (at a different frequency) back to the ground using another curved dish transmitter.
geostationary satellite
ground station
ground station
With three geostationary satellites placed in orbit around the equator worldwide communication is permitted with each satellite communicating with ground stations on different continents. In satellite television systems the signal from the satellite is broadcast over a wide area and collected by dish aerials on peoples homes.
Radio and Television Bands Band
Range
Use
Properties
ELF (Extra Low Frequency)
30 to 3,000 Hz
submarines
pass through water
LF (Low Frequency)
3 to 300 KHz
long distance radio communication
reflect off ionosphere
HF (High Frequency)
0.3 to 30 MHz
local radio communication
travel close to ground
VHF (Very High Frequency)
30 to 300 MHz
broadcast FM radio
short range
UHF (Ultra High Frequency)
300 to 3,000 MHz
television
travel in a straight line
> 3,000 MHz
satellites and mobile phones
pass through ionosphere
Microwaves
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Waves & Radiation Summary Notes
Section 3 - Infra-Red Infrared Infrared radiation is simply heat radiation with a wavelength longer than that of visible light. Infrared radiation is used by physiotherapists to speed up the recovery of injured muscles and tissues. Infrared radiation is also used in a thermal imaging camera where objects at different temperatures show up as different colours on a picture called a thermogram. Uses Of Infrared Medicine: Tumours are warmer than surrounding tissue and therefore show up on a thermogram. Night Vision Infrared can be used to amplify light in a low-light situation to enable video recording and image capturing. Meteorology Weather satellites use infrared technology to determine water temperature and cloud formations. Art History Infrared lights can be used to look under layers of painting to determine if there are older layers underneath.
Types of Thermometer A thermometer requires some measurable physical property that changes with temperature. Type of thermometer liquid-in-glass digital crystal strip rotary
Property that changes volume of liquid resistance of thermistor crystals have different melting points different rates of expansion in bimetallic strip
Infrared
voltage of thermopile
thermocouple
voltage produced
Liquid-in-glass Thermometer bulb
liquid (mercury or alcohol)
glass
narrow tube
scale
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The liquid inside the thermometer expands when it is heated. It’s volume increases and it rises up the tube. The liquid expands more than the glass
Waves & Radiation Summary Notes
Section 4 - Light Optical Fibres An optical fibre is a long, thin thread of glass through which light can travel. Light signals can travel along an optical fibre at very high speed (around 200,000,000 m/s). Optical fibres can carry telephone signals and modern telephone systems consists of both optical fibres and electrical cables.
Optical Fibres and Endoscopes In optical fibres only light is transmitted along the fibre, but not heat. The optical fibre is therefore said to transmit ‘cold light’.
Optical fibres are used in endoscopes to view the insides of a patient without the need for surgery. In an endoscope one bundle of fibres is used to carry ‘cold light’ down into the patient. A second bundle is then used to send the image back to the surgeon’s eye. The bundles are flexible and so can be moved around inside the patient.
Reflection
Advantages of Optical Fibres
Light can be reflected as shown below : Optical fibres have many advantages over electrical cables. These include :
mirror i
r
1) 2) 3) 4)
i = angle of incidence r = angle of reflection
normal
5) 6)
When light is reflected from a plane (flat) mirror it is found that the angle of incidence is equal to the angle of reflection. This is known as the law of reflection.
7) 8)
smaller in size cheaper to make lighter greater signal capacity (more information transmitted) higher signal quality less energy loss per km (less repeater station required) are not affected by interference not easily ‘tapped’.
However, light signals in optical fibres only travel at 200,000,000 m/s as opposed to almost 300,000,000 m/s for electrical signals in a wire.
Principle of Reversibility.
Optical fibres are also more difficult to join together than electrical cables
If the direction of a ray of light is reversed it will follow same path, but in the opposite direction
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Waves & Radiation Summary Notes
Section 4 - Using the Spectrum Lasers A laser is a very intense beam of light that carries a great deal of energy. Medicine Lasers are used for eye surgery, to remove birthmarks and to remove cancerous tumours. Welding and Cutting The beam of a laser can be focused to a microscopic dot of extremely high energy density for welding and cutting. Surveying and Ranging Helium-neon and semiconductor lasers have become standard parts of the field surveyor's equipment. A fast laser pulse is sent to a corner reflector at the point to be measured and the time of reflection is measured to get the distance. Barcode Scanners Supermarket scanners typically use helium-neon lasers to scan the universal barcodes to identify products. LASER
Operation of an Optical Fibre Optical fibres work because of total internal reflection. This is when all the light is reflected inside the glass and none escapes into the air. Total internal reflection occurs when the angle of the incident ray is greater than an angle known as the critical angle (about 42o for glass). In this way light can travel great distances through optical fibres without ever leaving the fibre
Operation of an Optical Fibre Optical fibres work because of total internal reflection. This is when all the light is reflected inside the glass and none escapes into the air. Total internal reflection occurs when the angle of the incident ray is greater than an angle known as the critical angle (about 42o for glass). In this way light can travel great distances through optical fibres without ever leaving the fibre
large angle of incidence
all light reflected along fibre
An Optical Fibre Transmission System In an optical fibre transmission system, such as a telephone system, electrical signals are converted into pulses of light by laser. These pulses of light then travel down the optical fibre and at the other end are converted back into electrical signals by a photodiode. The laser and photodiode are so fast acting that hundreds of different signals can be transmitted down the same fibre simultaneously. Page 14
Waves & Radiation Summary Notes
Section 5 - Light & Sight air
normal
Refraction
glass
Refraction is the bending of light as it passes from one material into another.
r i
When light passes from air into glass it bends towards the normal and when it passes from glass into air it bends away from the normal. i = angle of incidence r = angle of refraction
The Eye retina
Lenses A convex (converging lens) causes rays of light to be brought to a focus.
iris pupil
optic nerve
cornea lens
A concave (diverging lens) causes rays of light to spread apart.
Light is focused on the retina of the eye by the cornea and lens. Most of the refraction takes place at the cornea. The lens changes shape and allows the eye to focus on objects at different distances.
Focal length Focal length is the distance between a lens and the point where parallel rays of light are brought to a focus.
focal length
i = angle of incidence r = angle of refraction
The focal length of a convex lens can be measured experimentally by placing the lens in front of a screen and moving the lens until a sharp image of a distant object is obtained on the screen, then measuring the distance from the lens to the screen (this is the focal length).
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Waves & Radiation Summary Notes Image Formation in the Eye The eye forms an image which is upside-down (inverted) and back-to-front (laterally inverted).
The image is also smaller than the object being viewed.
Sight Defects Short-Sight
Long-Sight
People with short-sight are unable to focus on distant objects clearly. Short-sight can be corrected by using a concave lens.
People with long-sight are unable to focus on close objects clearly. Long-sight can be corrected by using a convex lens.
Short-sight occurs because parallel rays of light from a distant object are brought to a focus in front of the retina. (Image is short of retina)
Long-sight occurs because diverging rays of light from a nearby object are brought to a focus behind the retina (Image is long of retina)
Short-sight can be corrected by placing a concave lens in front of the eye to diverge the rays of light entering the eye
Long-sight can be corrected by placing a convex lens in front of the eye to converge the rays of light entering the eye
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Waves & Radiation Summary Notes
Section 6 - UV and X-rays X-rays X-rays can be used to find broken bones.
Ultraviolet Ultraviolet radiation is radiation with a wavelength shorter than that of visible light.
A beam of X-rays is aimed the patient. Since bone is more dense than the surrounding tissue it absorbs more X-rays. A ʻshadowʼ is therefore cast on the photographic film placed behind the patient. The X-rays blacken the film, so the bones show up white and any breaks as dark lines.
Medicine: Ultraviolet radiation is used to treat skin problems and to sterilise medical instruments. Forensics Forensic scientists will use ultraviolet lights to look for clues at a crime scene. CD/DVD Players CD and DVD players use lasers, which are a form of ultraviolet light, to read information off of the disc. Other uses for UV light include getting detecting forged bank notes in shops, and hardening some types of dental filling. Excessive exposure to ultraviolet radiation (e.g. from the Sun) can cause skin cancer.
arm muscle dark on plate X-ray source
llight
arm bone
photographic plate
Photographic film is frequently used as a detector of X-rays.
Computerised Tomography
Industrial Imaging Welders use X-ray technology to inspect tightly welded seams that appear perfect for bubbles and other anomalies.
In computerised tomography a series of horizontal X-ray images are taken of the body. A computer then uses these ‘slices ‘ to build up a 3-dimensional (3-D) picture of the body. This enables small tumours to be found very accurately and provides more detailed images than a conventional X-ray pictures.
Astronomy X ray telescopes study high-energy X-rays coming from exotic sources such as black holes, exploded stars and the hot gas in galaxy clusters.
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Health Physics Summary Notes
Section 7 - Nuclear Radiation Radiation can kill or damage living cells.
Uses of Radiation
Nuclear radiation is used in medicine to sterilise medical instruments by killing germs. It can also be used to kill cancerous cells either by placing an alpha source next to the tumour or by firing a beam of gamma rays at the tumour from a variety of different directions (therefore only the tumour receives a large radioactive dose and the surrounding tissue is relatively unharmed). Radiation is also used for diagnosis. A radioactive tracer (a gamma source) is injected into the patient. The tracer is carefully chosen so that it will collect in the organ being studied and an image of the organ can be then be taken with a gamma camera.
Smoke alarms contain a weak source made of Americium-241. Alpha particles are emitted from here, which ionise the air, so that the air conducts electricity and a small current flows. If smoke enters the alarm, this absorbs the a particles, the current reduces, and the alarm sounds.
Thickness control
In paper mills, the thickness of the paper can be controlled by measuring how much beta radiation passes through the paper to a Geiger counter. The counter controls the pressure of the rollers to give the correct thickness. Sterilising Even after it has been packaged, gamma rays can be used to kill bacteria, mould and insects in food. This process prolongs the shelf-life of the food, but sometimes changes the taste.
The Atom An atom is the smallest particle into which matter can be divided. It is made up of a central nucleus with orbiting electrons. electrons
Types of Radiation nucleus containing protons and neutrons
The nucleus contains positively charged protons and uncharged (“neutral”) neutrons. The nucleus makes up nearly all the mass of the atom and contains all the positive charge The electrons orbit the nucleus at high speed. They are negatively charged and much lighter (1/2,000 th) than neutrons or protons. An atom is normally electrically neutral as it has the same number of negative electrons orbiting the nucleus as positive protons in the nucleus.
There are three types of nuclear radiation. These are alpha (α), beta (β) and gamma (γ) Whenever radiation passes through a material (medium) some of it’s energy is absorbed by the material. The amount of absorption depends on the type of radiation and the material it is passing through. Type
Range in Air
Absorbed by
α
20 cm of air
sheet of paper
β
a few metres
2-3 mm Aluminium
γ
not absorbed
2-3 cm Lead
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Waves & Radiation Summary Notes Activity
Half-life
The activity of a radioactive source is the number of atoms that decay each second (number of radioactive particles released each second).
The half-life of a radioactive source is the time taken for it’s activity to half.
The activity of a radioactive source is measured in Becquerels, Bq.
The half-life of a radioactive source can be measured by taking measurements of the activity of the source at regular intervals of time using a Geiger-Muller tube and counter. source
The activity of a radioactive source decreases with time.
Geiger-Muller tube
counter
Half-life calculations Example 1: If a 8,000 Bq source of activity has a half-life of 6 days what activity will it have 18 days later ? 18 days 1
8,000
Background activity is then subtracted from each reading and a graph of activity against time is drawn. activity
= 3 x 6 days = 3 half-lives 2
4,000
time
2,000
3
1,000
The activity after 18 days is 1,000 Bq
The time taken for the activity to half can then be established from the graph
Safety
Example 2: Calculate the half-life of a source that decreases in activity from 32 kBq to 8 kBq in 24 days. 32
1
16
2
Safety precautions need to be taken when handling radioactive materials. These include : • • • • •
8
2 half-lives = 24 days 1 half-life = 12 days The half-life of the source is 12 days
Always handle with forceps Point source away from body Store in lead container Label all sources Wash hands after use
Dose equivalent The dose equivalent of a radioactive source is a measure of the biological risk of the source and is measured in Sieverts, Sv.
The biological effect of radiation depends on: • the type of absorbing tissue • the type of radiation • the total energy absorbed
The dose equivalent takes into account the type of radiation and the total energy absorbed. Page 19
Waves & Radiation Summary Notes The biological effects of radiation All ionising radiation can cause damage to the body. There is no minimum amount ofradiation which is safe. The risk of biological harm from an exposure to radiation depends on: •the absorbed dose •the kind of radiation •the body organs or tissue exposed. The body tissue or organs may receive the same absorbed dose from alpha or gammaradiation, but the biological effects will be different. To solve this problem a radiation weighting factor,WR is used which is simply a number given to each kind of radiation as a measure of its biological effect. Some examples are given below . WR Type of radiation 1 10 20
beta particles / gamma Protons and Fast Neutrons alpha particles
Absorption of radiation When considering the radiological effects of radiation, we must take into account not only the total energy absorbed but the mass of the material within which it is absorbed, e.g. for a dental X-ray, the absorbing mass would be the mass of tooth, gum, jaw and cheek.
Absorbed dose, D, is the energy E absorbed by unit mass. Absorbed dose is measured in grays. 1 gray (Gy) = 1 joule per kilogram.
D=E m
The absorption of energy by a substance depends on:
o the nature and thickness of the substance o the type of radiation o the energy of the particles or photons of the radiation. Alpha radiation is absorbed within a fraction of a mm of tissue, which gives a very high absorbed dose because of the small absorbing mass.
Dose equivalent When scientists try to work out the effect on our bodies of a dose of radiation they prefer totalk in terms of dose equivalent. The dose equivalent H is the product of D and WR. Dose equivalent = absorbed dose x radiation weighting factor
H = DWR
The dose equivalent is measured in sieverts, Sv. Example A worker in the nuclear industry receives the following absorbed doses in a year. 30 mGy from gamma radiation, WR = 1 300 mGy from fast neutrons, WR = 10 Calculate the dose equivalent for the year. H = DWR for gamma H = 30 x 10-3 x 1 = 30 x 10-3 Sv for neutrons H = 300 x 10-6 x 10 = 3.0 x 10-3 Sv total H = 30 x 10-3 + 3.0 x 10-3 = 33 x 10-3 Sv Page 20
Waves & Radiation Summary Notes Background radiation Everyone is exposed to background radiation from natural and from man-made radioactivematerial. Background radiation is always present. Some of the factors affecting backgroundradiation levels are: • Rocks which contain radioactive material, expose us to ionising particles • Cosmic rays from the sun and outer space emit lots of protons which causeionisation in our atmosphere • Building material contain radioactive particles and radioactive radon gas seeps upfrom the soil and collects in buildings, mainly due to lack of ventilation. • The human body contains radioactive potassium and carbon • In some jobs people are at greater risk. Radiographers exposed to X-rays used in hospitals and nuclear workers from the reactor. Natural radiation is by far the greatest influence on our exposure to background radiation.
NUCLEAR REACTORS Advantages of using nuclear power to produce electricity •Fossil fuels are running out, so nuclear power provides a convenient way ofproducing electricity. •A nuclear power station needs very little fuel compared with a coal or oil-firedpower station. A tonne of uranium gives as much energy as 25000 tonnes of coal. •Unlike fossil fuels, nuclear fuel does not release large quantities of carbon dioxideand sulphur dioxide into the atmosphere,which are a cause of acid rain. Disadvantages of using nuclear power to produce electricity •A serious accident in a nuclear power station is a major disaster. British nuclearreactors cannot blow up like a nuclear bomb but even a conventional explosioncan possibly release tonnes of radioactive materials into the atmosphere. (TheChernobyl disaster was an example of a serious accident.) •Nuclear power stations produce radioactive waste, some of which is very difficultto deal with. •After a few decades nuclear power stations themselves will have to be disposed of. Page 21
Waves & Radiation Summary Notes Nuclear fissionAn atom of uranium can be split by a neutron. This can produce two new nuclei plus theemission of neutrons and the release of energy.
Chain reaction Once a nucleus has divided by fission, the neutrons that are emitted can strike otherneighbouring nuclei and cause them to split releasing energy each time. This results in whatis called a chain reaction as shown below.
Nuclear fusion Fusion occurs when two light nuclei combine to form a nucleus of larger mass number, e.g. the fusion of two deuterium nuclei (an isotope of hydrogen) to form helium.
Once again, there is a decrease in mass in the fusion process and the energy released is produced as kinetic energy of the fusion products. The energy released by the sun and other stars is produced by nuclear fusion.
In a controlled chain reaction, on average only one neutron from each fission will strikeanother nucleus and cause it to divide. This is what happens in a nuclear power station. In anuncontrolled chain reaction all the neutrons from each fission strike other nuclei producing alarge surge of energy. This occurs in atomic bombs. Page 22
