9/22/20
Electromagnetic Spectrum
a) State that all electromagnetic waves are transverse waves that travel with the
same high speed in vacuo and state the magnitude of this speed
b) Describe the main components of the E.M. Spectrum c) State examples of the use of the following components:
i. radio waves (e.g. radio and television communication)
ii. microwaves (e.g. microwave oven and satellite television) iii. infra-red (e.g. infra-red remote controllers and intruder alarms)
iv. light (e.g. optical fibres for medical uses and telecommunications)
v. ultra-violet (e.g. sunbeds and sterilisation)
vi. X-rays (e.g. radiological and engineering applications)
vii. gamma rays (e.g. medical treatment)
d) Describe the effects of absorbing electromagnetic waves, e.g. heating,
Tips
They can travel through vacuum.
All electromagnetic spectrum travel at 3x108m/s through vacuum.
They are all transverse waves.
They transfer energy.
They can be emitted and absorbed by matter.
They are emitted when charged particles oscillate or lose energy.
– E.G. Vibrating atoms in the bulb filament gives out light and infra-red.
Radio Waves
Generated by radio transmitter, a high frequency electric current in a
transmitting aerial.
Detected by metal aerials and tuned circuits.
The longer waves will bend around obstacles
Used in
– Radio, TV and satellite communication.
– Microphone controls the current such that the radio wave pulsate.
– Ultra high frequency (TV) and very high frequency (radio)
– UHF and VHF do not bend, so required direct path between
Microwaves
Shortest wavelength of all radio waves
Generated by microwave communicating dish/ klystron tube
Detected by receiving aerials
Used for
– Carrying TV and telephone signals between dish aerials
and satellites.
– Radar detection as metal reflect the waves strongly. Radio
detection and ranging (RADAR)
– Cooking (microwave oven)
Water absorbed microwaves of a particular frequency.
Since EM waves carries energy, the energy is absorbed
by the water.
Microwaves penetrate into the food to heat up
ÞSatellite links.
A microwave beam, carrying television pictures, can be aimed at a satellite positioned between two continents. The satellite receives and amplifies the beam before sending it back to receiving aerials on the ground. ‘Live’, world-wide TV coverage is achieved in this way.
Ethical Issues:
Þ
There is a lot of suspicion that microwaves can cause
tumours and cancers in the same way that, for example,
frequent drinking can cause liver cancer.
Þ
Cellular phones are the largest problem of all microwave
technology. Having one phone right next to the ear for ten
minutes to an hour, means that the brain and inner ear are
being bombarded by microwave energy.
Infra-red
Sources include the sun and any hot objects.
Most radiate a wide range of wavelength. Travels in a straight line
without a loss of heat to the air. When absorbed, the temperature will rise.
When objects heats up, it emits more infrared waves with shorter
wavelength. At about 700°C (red hot) wavelength can be detected by the eye. About 1000°C, whole spectrum can be seen.
• Used in
• heating appliance such as heaters or oven toaster. Heat is generated when
electricity pass through the heating coil.
• remote controllers to pass information from the controller to the appliance.
• "Passive Infra-Red" (PIR) detectors are used in burglar alarm systems, These
detect the Infra-Red emitted by people and animals.
• Weather forecasters use satellite pictures to see what's heading our way, some
taken using IR cameras, because they show cloud and rain patterns more clearly.
• Because every object gives off IR waves, we can use them to "see in the dark".
Visible Light
Sources include the sun, lamps and hot objects.
Detected with eye, photographic film, light dependent resistor.
Essential for photosynthesis.
Used in fiber optics for:
– Communications, transmission of signals across distance
– Medical use, used in endoscopes to view inaccessible parts
– Medical use, laser for welding detached retina, sealing blood
vessels.
Ultraviolet Light
Sources include the sun, mercury vapour lamps / ultraviolet lamps.
Detected with photo film, fluorescent chemicals.
Absorbed by glass, damages and kills living cells.
Used in
– Sun beds, ultraviolet lamps within gives a tan
– Detection of forgeries (banknote, signatures), glows when exposed
to ultraviolet light
– Fluorescent tubes, fluorescent powder gives off white light when it
absorbs ultraviolet rays. Works with some washing powder too.
X-ray
Produce in x-ray tubes, when fast-moving electrons lose energy
quickly. X-ray is emitted when electrons hits a metal target. Detected with photographic plates, fluorescent screens
Short wavelength x-rays are penetrating and dangerous. Long-wavelength x-rays are less penetrating
Used in
– Hospital use
X-ray radiography, long-wavelength x-ray are able to penetrate skin but
not bones. Structure of bone can be seen, (and other extra-stuff too).
Medical treatment, concentrated beams of x-ray can be used to kill
cancer cells.
– Engineering application
Inspection of welds joins or pipes, x-ray photos to show possible flaws
inside metals.
Gamma Rays
Smallest wavelength and the most energy.
Generated by radioactive atoms (cobalt 60) and in nuclear explosions,
when nuclei of unstable atoms break up or lose energy.
Detected by photographic plate, GM tube and in cloud chamber
Penetrating, able to kill living cells, including cancerous cells
Used in
– Medical treatment, treatment of cancer cells (radiotherapy).
– Medical treatment, radioactive substance used as tracers. Patient
with lung problem can breath in xenon-133 ( it releases gamma rays). A special gamma camera uses the rays to build up a picture.
Effect of absorbing EM waves
Damage to the cells (living tissues) in our body.
– Damage is due to ionisation
cells may turn cancerous and start multiplying uncontrollably.
Cells may not function normally
– Damage due to heating effects
If levels of radiation are high, the body may absorb the energy from the
radiation
Cells will die. Eg. sunburn