The Electromagnetic Spectrum by year 12 physics students


Thanusiyan Jekanathan 12Y

Microwaves are a form of electromagnetic radiation with wavelengths ranging from as long as one meter to as short as one millimetre, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz.

The existence of radio waves was predicted by James Clerk Maxwell in 1864 from his equations. In 1888, Heinrich Hertz was the first to demonstrate the existence of radio waves by building a spark gap radio transmitter that produced 450 MHz microwaves, in the UHF region. In 1894 Indian radio pioneer Jagdish Chandra Bose publicly demonstrated radio control of a bell using millimetre wavelengths, and conducted research into the propagation of microwaves.


Sir Jagadish Chandra Bose, (30 November 1858 – 23 November 1937) was a Bengali polymath, physicist, biologist, botanist, archaeologist, as well as an early writer of science fiction.

The first practical microwave equipment was built in the 1930’s. Its development was spurred during World War II by research for improving radar. By 1940, the British had installed a chain of radar installations along the coastline to warn against air attack.

After World War II, funding for further development in the microwave field was reduced drastically, and research and development efforts over the next 40 years were tied to fluctuations in defence budgets, with the search for commercial applications occurring generally during low periods in government demand. Applications developed included communications, commercial radar, industrial heating, and industrial measurements.

Microwaves are generated using either semiconductor devices or electron tubes such as the magnetron (used in microwave ovens) and the klystron. For transmission over a short distance, microwaves are sent through a coaxial cable or through a hollow metal tube called a waveguide. For long distances, microwaves are transmitted by a parabolic or horn-reflector antenna that is usually designed to direct the microwaves in a relatively narrow beam.


High-power N-channel field-effect transistor


High-power klystron used for spacecraft communication at the Canberra Deep Space Communications Complex.

All warm objects emit low level microwave radiation, depending on their temperature, so in meteorology and remote sensing microwave radiometers are used to measure the temperature of objects or terrain. The cosmic microwave background radiation (CMBR), is a weak microwave noise filling empty space which is a major source of information on cosmology’s Big Bang theory of the origin of the Universe.

Microwaves are reflected by metal surfaces. They heat materials if they can make atoms or molecules in the material vibrate. The amount of heating depends on the intensity of the microwave radiation, and the time that the material is exposed to the radiation. They pass through glass and plastics. They pass through the atmosphere. They pass through the ionosphere without being reflected. They are absorbed by water molecules, how well depends on the frequency (energy) of the microwaves. Transmission is affected by wave effects such as reflection, refraction, diffraction and interference.

Microwaves are used in:

Medicine In some cases microwaves relieve symptoms better than drugs. The use of microwaves is safer than surgery. For example, the wave can heat enlarged prostates, therefore killing extra tissue.

Industry Microwaves can decrease air pollutants, disinfect hospital waste, improve dry cleaning solvents, and clean up contaminated soil.

Science Satellite communications have used microwaves in the past. Scientists use microwaves to make chalcopyrite, a semi-conductor, made of copper. Microwaves can also be used to remove materials, and chemicals that can be used for analysis.

Consumer goods Microwave ovens are used to heat and re-heat food evenly by agitating water molecules in food when they absorb microwaves of a certain frequency.


The water in living cells can also absorb microwave radiation. As a result, they can be killed or damaged by the heat released. Some burglar alarms send continuous microwaves (short radio waves) into an area. Any movement in that area disturbs the wave pattern and sets off the alarm.

Communication: Microwaves are used in mobile telephones, telegraph, television, and satellites. Satellite TV and mobile phone (or smartphone) networks use microwaves. Microwave transmitters and receivers on buildings and masts communicate with the mobile telephones in their range.


Certain microwave radiation wavelengths pass through the Earth’s atmosphere and can be used to transmit information to and from satellites in orbit.


Microwaves do not contain sufficient energy to chemically change substances by ionisation, and so is an example of non-ionising radiation. It has not been shown conclusively that microwaves have significant adverse biological effects at low levels. Some, but not all, studies suggest that long-term exposure may have a carcinogenic effect. This is separate from the risks associated with very high intensity exposure, which can cause heating and burns like any heat source, and not a unique property of microwaves specifically. When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in the body. Exposure to microwave radiation can affect the testes and produce cataracts by this mechanism, because the microwave heating denatures proteins in the crystalline lens of the eye (in the same way that heat turns egg whites white and opaque). The lens and cornea of the eye are especially vulnerable because they contain no blood vessels that can carry away heat. Exposure to heavy doses of microwave radiation (as from an oven that has been tampered with to allow operation even with the door open) can produce heat damage in other tissues as well, up to and including serious burns that may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content.

The World Health Organisation has stated that “to date, no adverse health effects have been established as being caused by mobile phone use”. Another area of concern is the radiation emitted by the fixed infrastructure used in mobile telephony, such as base stations and their antennas, which provide the link to and from mobile phones. This is because, in contrast to mobile handsets, it is emitted continuously and is more powerful at close quarters. On the other hand, field intensities drop rapidly with distance away from the base of transmitters because of the attenuation of power with the square of distance. Therefore they are not considered a danger.

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