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Electromagnetic
~waves~ (Communications (Radio Communications
Carrier…
Electromagnetic
~waves~
EM Spectrum
Definition
electric & magnetic disturbances for energy transfer
from an energy source to an absorber; used everyday
Microwave ovens = microwave source energy trans. to food
Radiant heater = Infrared (IR) trans. to heat surroundings
Don't transfer matter, energy trans. depends on λ
Diff. λ = diff. effects; 10km (radio) to 1 picometre (gamma)
Eyes detect visible light, ltd part of EM spectrum (400-700 nm)
Speed of EM waves
All EM waves travel at 3.0x10^8 m/s thru vacuum
velocity = frequency x wavelength v = fλ
Speed and Frequency
Wave speed doesn't change in same medium
If λ decr. = freq. incr, wave energy incr. w/ freq
Therefore: λ decr. = Energy, freq. incr.
Light, Infrared,
Micro & Radiowaves
Light
Ordinary lamp light = white light
(All colours of visible spectrum in it)
λ decr. from red to violet; spectrum seen in rainbow
Or spectrum seen when split by glass prism
Photography: understand shades, colours affect photo
Film - Light focused by lens to light-sensitive film; later
dev. to see image of photographs
Digital/Mobile - light focused by lens onto sensor =
Ks of tiny light-sensitive cells (pixels); 1 pixel = 1 dot
Image can be seen on screen, photos stored electronically w/ memory card
IR
Hotter objects emit more IR - all objects emiti it
Absorbed by skin; damages, burn, kill skin cells w/ heating
Devices - FO, absorbed less by glass fibre than visible light
Remote controls transmit signal w/ IR, buttons = sequence
of IR pulses; easy prod, detected electronically
IR scanners in medicine detect IR hot spots on body
normally means unhealthy tissue; IR cameras = night vision
E.heaters warm quicker rooms; E.cookers w/ halogen bulbs heat food faster than standard hobs
Microwaves
Shorter λ than radiowaves, for communications & heating
Satellite TV, waves pass atm & reach satellites, also mobiles
Ovens, heat faster than standard - penetrate food, absorbed
by H2O molecules inside, actual oven doesn't become hot
Radiowaves
Frequency range = 300,000Hz to 3,000MHz
Carry radio, TV, mobile signals; replace cables
connects printers, mouse (Bluetooth, 10m range)
Micro/Radiowaves dangerous - penetrate bodies
heat internal parts
Communications
Radio Communications
Carrier waves - waves to carry any type of signal
Phones use Radio.w to send signals to nearest phone mast
Can be Radio, Micro, IR, Visible - depends on distance
and capacity needed (e.g. Micro for satellite,internat. connect)
Radio Wavelengths
Radio/Microwaves spectrum put into bands of λ
Shorter waves = more info, shorter range (incr. atm absorb),
less spread out; diff. λ = diff. comm. purposes
Micro - Satellite phone, TV links/satellite broadcasting
(They can travel to space, spread out less = stronger signal)
Radio (>100m) - Nat/Internat radio stations (longer range)
Radio (>1m) - Local radio stations (for emergencies, ltd area)
Radio (<1m) - TV broadcasting w/ TV masts (carry more info)
Phones & EM radiation
Phone sends radio signals; radiation may affect brain
if too close, espec. w/ children (thinner skulls)
May 2000 - recommedn ltd use of technology by kids
More research needed
More about signals & carrier waves
To speak = prod. sound waves w/ diff. freq's
Vary (i.e. modulate) Amplitude & Freq. of what you prod.
Radio station: Microphone prod. AC current called audio
signal when sound waves reach
FO Communications
FO are very thin glass fibres, transmit signals w/ light/IR
Light can't escape, reflect along surface of fibres
FO carry much more info, much shorter λ, carry more pulses
FO are more secure, signals stay in fibre
From mic to speaker
OSCer supplies carrier waves to transmitter in AC
Audio is supplied to trans'er to modulate carrier waves
Modu'd carrier waves supplied to trans'er aerial, induce AC
causes OSC in receiver, freq. for OSC = freq. of radio.w
Receiver separates audio from carrier, audio given to speaker, sends sound similar to what's received by mic
Ultraviolet, X-rays,
Gamma rays
Ultraviolet (UV)
Some chem's emit light w/ absorb of UV
Posters, ink glow if containing chem's
Inc. security pens (mark valuable objects)
Carry more energy, shorter λ than Visible; in sun, sunbeds
Can cause blindness; too much directly = sunburn, cancer
Use sun cream outdoors block UV from sun
X-rays & Gamma
Only stopped by dense thicc substances
Thicc piece of lead will stop 'em; similar properties
Both have short λ, much more energy/s than longer-λ
Differences: Gamma from RA substance, shorter λ
Using Gamma
Harmful Bact: 20% of food lost w/ spoilage (bact.)
Causes food poisoning; gamma exposure kills 99%
inc. Salmonella (in poultry) & Clostridium (causes botulism)
Exposing surgical instruments sterilises stops infect spread
Cancer Cells: Doctors, medical physicists destroy tumours
Narrow beam of G from RA source (Co-60) direct at tumour
Beam is aimed from diff. directions kill tumour where rays meet
Safety matters
X-rays, G-rays pass can knock e- in substances
Atoms become charged w/ ionisation = ionising radiation
Ionisation happens w/ living cells, can damage/kill cell
Thus too many G/X-rays can cause cancer; hi dose kills
living cells, low dose = gene mutation & cancerous growth
People who use equipment/substance prod'ing ionising rad.
Must wear film badge, if badge shows over-exposure,
wearer needs break for a period of time
X-rays in Medicine
Wut?
Used w/ broken bones, EM waves on the short-λ end
Prod. in X-ray tube when fast e- hit target; λ is same
as atom diameter; Make radiograph/X-ray photo
Light-proof cassette w/ photographic film/flat-panel detector
placed on other side of patient
Process
When switched on, X-rays pass thru part under investigation
Passes soft tissue, absorbed by bones, teeth, metal
Parts of film X-ray reach are darkened; negative bone image
Hole/Cavity = dark in image of bright white teeth
Contrast medium (absorbs X easily) for organ radiographs
Ba meal given before radiograph on stomach
Holes in Pb plating allows X-rays to reach w/in gaps
Flat-panel detector = small screen w/ CCD - charge-coupled device' sensor in CCD convert X to light, create e.signals in sensor sent to comp; display digital X image
Radiation dose
X, G and RA rad. all ionise substances
All ionising rad. is dangerous; dose = measure of damage
damage, type, length of exposure, energy/s absorbed
A rad. is x10 more damaging than X; meas. in sieverts (Sv)
No evidence of safe limit; always low level from background
Cosmic radiation (space), radon gas seeps from deep U-ground
X-ray Therapy
Doctors use this to destroy cancerous tumours
Thicc plates prevents X-ray reaching healthy tissue
Gap between plates allows X-rays thru, shorter-λ used
X-rays have much more energy, lo-energy pass soft tissue, absorbed by bones & teeth; more energy needed to destroy