Please enable JavaScript.
Coggle requires JavaScript to display documents.
Electromagnetic Radiation (gamma rays (wl 100 billionths of a metre, only…
Electromagnetic Radiation
sound, light, and heat transmit energy via waves
different forms of electromagnetic radiation make up electromagnetic spectrum
300 000km/s (speed of light)
travels through empty space, gases, liquids, and some solids
when substance absorbs electromagnetic radiation, it also absorbs its energy
substance will heat/change
as energy increases, frequency increases, wavelength decreases
wave motion
longitudinal (e.g. sound waves)
particles vibrate back and forth in same direction as (parallel to) wave
transverse (ocean, slinky, rope)
particles vibrate last right angle to wave
wave properties
frequency: number of waves produced per second
hertz (Hz)
wavelength: distance between two successive waves
metres
amplitude: maximum distance wave extends beyond middle position
larger the amplitude, the more energy the wave carries
the wave equation
if the frequency increases, the the wavelength will decrease
v= fλ
v = speed (m/s)
λ = wavelength
f = frequency
electromagnetic radiation
when electric charges move, a magnetic field is generated around wire
generates corresponding electric field
endless loop of electric/magnetic waves
visible spectrum
rainbow of colours that combine to form white light
only small band of electromagnetic spectrum (that we can see)
1666: Isaac Newton passed narrow beam of light through glass prism
created rainbowwww
findings:
white light = all colours of visible spectrum
red, orange, yellow, green, blue, indigo, violet
each colour has different wave and frequency
red bends least (700nm), violet bends most (400nm)
1nm (nanometre) = 1 x 10^-9
apx 1000 waves fit into 1mm
object looks specific colour because it absorbs all the other colours of light
e.g. apple absorbs orange, yellow, green, blue, indigo, violet
reflects red light
therefore you see red
black absorbs all light
white reflects all light
things look different under different coloured light
additive mixing (light)
blue, green, red
secondary colours:
blue + green = cyan
primary colours can make white light:
blue + red = magenta
red + green = yellow
vision
3 types of cone cells
each type sensitive to 1 primary colour
combinations of signals give full colour
filters
absorbs all other colours , lets one colour through
e.g. red filter: only lets red through, absorbs all other colours
e.g. yellow lets red & green through, absorbs all other colours
used in 3D film, photography, theatre, etc
scene filmed from two angles
projected in two different colours
two different colour lenses (filters)
can also use polarising glasses
only allow waves that vibrate in certain a direction (one plane of vibration)
other-direction-waves are absorbed
interpreted as different view --> 3D vision
subtractive mixing (printing, paint)
primary colours can make black:
cyan, magenta, yellow
as more pigments added, more colours can be absorbed
secondary colours:
cyan + magenta = blue
magenta + yellow = red
cyan + yellow = green
printing
photo separated into 4 colour plates
cyan
yellow
magenta
black (increase contrast)
together form full spectrum of colour, mix dots on top of each other
dispersion
splitting of white light into colours
e.g. rainbow
sunlight enters water droplets, dispersed into individual colours
total internal reflection, reflects one colour back out
raindrops at different heights, you see different colours
secondary rainbow
fainter, higher in sky
reversed colours
light reflected twice
scattering
light interacts with particles in atmosphere
higher frequency high energy light waves: more easily scattered
blueish light scattered more than reddish light
blue sky
red sunset: longer distance through thicker layer of atmosphere, blue already scattered
radio waves
used by TVs and radio to transmit signal
make electrons in antenna vibrate
converted into sounds/images
produced through vibrating/oscillating electrons in a transmitting aerial
can travel large distances
short waves (wl 30m) signals beamed upwards at angle
waves reflected by ionosphere to far-away location
communications
long waves bend (diffract) around earth
microwaves
pass through ionosphere, reflected by satellites
line-of-sight links
different radio stations broadcast at different frequency
received when your radio tunes to same frequency
sound wave converted to electric signal
signal amplified
oscillator adds carrier wave
frequency/amplitude modulated
radio mast transmits signal
radio antenna receives signal as vibrations
amplified and carrier wave removed (demodulation)
1 more item...
AM (amplitude modulated)
same frequency, amplitude varies
wl >100m
can bend around large obstacles (diffract)
travel further
lower quality (more interference)
FM (frequency modulated)
same amplitude, frequency varies
wl 3m
microwaves
shorter wavelengths (than radio)
radar & communications systems, cooking (shorter waves = wl 0.1mm)
microwaves absorbed by water, sugar, & fats in food
molecules vibrate and heat up
glass, paper, and most plastics don't absorb
metal reflects
infrared radiation
heat from sun
'infra' = below --> below red light on spectrum
cannot see, only feel as heat
all objects above absolute zero (-237) emit infrared
hotter the object, more infrared emitted
detected using infrared camera
modern communications networks
analogue signals
copper landlines
voltage determined by speaker's voice
limited - signal loss and interference
digital signals
tv, internet, mobile phone, music
modem connects computer to phone line
converts digital computer signal into analogue (modulation: coding signal into different format)
transmits
if interference, still can be read
binary number system
increasing capacity
before; phone signals sent through insulated twisted-pair copper wires
600-pair copper cable can carry 600 two-way conversations
inexpensive, minimal signal loss over short distances
now: coaxial cables
linked underground since 1960s
2 cables of 50 tubes can carry 2700 two-way conversations
repeaters every 45m to reinforce signal
FDM (frequency division multiplexing): different signals sent at same time w diff frequencies
[for digital signals] TDM (time division multiplexing)
data broken into chunks, woven together
transmitted at single frequency as data stream
FDM and TDM can be used together
bandwidth: amount of data that can be carried by channel
optical fibre
1980s
thin, flexible silica glass/plastic tubes
central core (pure silica glass)
surrounded by cladding (less dense glass)
coated with plastic (minimise interference)
laser light produces narrow beam of coherent light waves
coded as binary, sent in flashes
totally internally reflects down tube
optical receiver converts to digital signal
can transmit much higher frequency signals (than copper wire) --> greater bandwidth (30 000 calls/fibre)
bandwidth can be extended by sending waves of different wavelength at same time
also lighter & more flexible
signal can be transmitted 200km without loss, overall less interference
cable made up of many single fibres
incoherent light (normal):
range of frequencies
many wavelengths
out-of-step waves
coherent light (laser):
one frequency & wavelength
in-step waves
microwave links
short, straight-line paths
2000 conversations
repeater towers every 50km
long-range mobile communications, remote areas
mobile phone networks
digital signal of voice through microwaves
interconnected base stations: antennas receive and transmit mobile signals
each base station in centre of hexagonal cell
each cell uses diff frequency to transmit signal
no adjacent cells use same frequency
when you dial, base station that receives strongest signal directs call to telephone exchange
exchange directs call to base station of receiving phone
internet
router: manages connection between computer & internet server
makes sure your message reaches destination
data (file/email) first split into packet (apx 1500 bytes)
travel over 'packet switching network' -- directed along best path
reassembled once reaches destination
wireless internet
transmitting internet connection using radio waves
wireless alternative for internet within LANs (local area networks) but not long-range coaxial cable/optical fibres
range = 30m indoors, 100m outdoors
ultraviolet light
'ultra' = beyond
beyond violet light
sunlight: UV + infrared + visible
body needs some UV to produce vitamin D
can tan/burn skin -- skin cancer (e.g. melanoma)
eye cataracts
therefore use sunglasses and sunscreen to protect
Bureau of Meteorology - daily UV index forecasts
fluorescence: when something absorbs UV light and emits visible light
e.g. white paper, teeth whiteners. laundry powders
sterilises
x-rays
great penetrating power
investigate structure of objects
find flaws in metals
can damage cells and tissues, genetic material inside cells
produced when electrons hit metal surface
radiology - produce image of bones
radiotherapy - kill cancer cells
CT (computed tomography) scan: detectors rotate around person, info analysed by computer to create images of organs
workers: protective lead shields, personal radiation monitoring device (x-rays, gamma rays, neutrons, beta particles) -- value must remain below certain point to protect workers
baggage inspection
low-medium energy x-rays
pass more easily through less-dense items
sensors behind detect variation of penetration through sample
different densities displayed as diff colours, builds image of diff items
gamma rays
wl 100 billionths of a metre
only stopped by thick sheet of lead/concrete wall
produced in making nuclear power/bombs
detected with photographic film/Geiger counter
high energy allows them to interact with matter
can free electrons from atoms --> ions
ions then ionise other atoms
used in radiotherapy to kill cancer cells
PET (position emission tomography)
injected with short-lived radioactive material
gamma rays emitted detected by PET scanner/camera
converted into 3D image
gives metabolic info, allows doctors to study how parts of a patient are functioning
