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- Describe the main features of the electromagnetic spectrum
The electromagnetic spectrum is the series of waves that are created by disturbances in electric and magnetic fields. They are arranged in order of increasing frequency and decreasing wavelength. Every wave on the spectrum is technically ‘light’; they all travel at lightspeed, and only a small part of the spectrum is visible to us.
- State the approximate value of the speed of all electromagnetic waves in vacuo.
All electromagnetic waves travel at the speed of 300,000,000 m/s in vacuum.
That is, 3 x 108 m/s.
- Describe the role of electromagnetic waves in:
• radio and television communications (radio waves)
• satellite television and telephones (microwaves)
• electrical appliances, remote controllers for televisions and intruder alarms (infra-red)
• medicine and security (X-rays).
Radiowaves have the longest wavelengths and so are used in radio and television communications. The long wavelengths and low frequencies of radiowaves reduce how easily they are absorbed and scattered, allowing them to be transmitted long distances. It also allows them to be reflected from the Earth’s ionosphere (a layer in the atmosphere) so it can travel even further.
Microwaves are used in satellite television and telephone communications. The higher frequency of these waves allow them to transmit more information, allowing higher quality communications. It also means that they cannot be reflected by the ionosphere – they pass through it.
Infra-red waves are used in electrical appliances like remote controllers, televisions and burglar alarms. They are also used in optic fibres. Note that infrared waves are also known as the ‘heat carriers’ – most hot objects emit infrared waves and infrared waves are absorbed by the surface of most objects too, causing them to warm up. As infrared waves are easily absorbed, and can’t penetrate many things, they are used in numerous systems – e.g. in burglar alarms, a beam of infrared is directed at a receiver. When the alarm is on, if something intercepts the beam, it no longer reaches the receiver, causing a signal to go off and the alarm to be activated.
Visible light waves are the light waves emitted by bulbs, lamps etc that is visible to us. Visible light is also used in optic fibres.
Ultraviolet waves are emitted by the sun, among other sources, and exposure to it can cause skin diseases. There is one type of UV light that is safer and often used in tanning booths. Many fluorescent substances only fluoresce upon being illuminated by UV waves too. Therefore, they are often used to detect forged bank notes (real ones often have markings only visible under UV light), as security markers (again, markers only visible under UV light, that tells us that the object it is present on is authentic and not a fake), etc. UV light is also used to disinfect water.
X-rays are used in imaging – they are used in security check when scanning luggage, for medical diagnoses, etc.
Gamma rays are produced by radioactive materials. It has a high frequency and so can penetrate through most materials and cause gene mutation and cancer. Alternatively, when controlled, it can be used in the sterilisation of medical equipment. It can also be used to detect the presence of cancer and its treatment.
- Demonstrate understanding of safety issues regarding the use of microwaves and X-rays
Direct exposure to X-rays can cause gene mutations – electrons can be knocked off of atoms, causing unwanted reactions, etc. Sometimes, the reactions are harmless or cause mutations that cause no change in the operation of the cell (these mutations are called silent mutations). On the other hand, if the mutation is serious enough, it may kill cells, cause cancer, etc.
Microwaves carry kinetic energy and will cause particles, mainly water, to vibrate faster, increasing temperature. Thus microwaves can cause internal heating of body tissues.
Notes submitted by Lintha and edited by Sarah.
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