Science/Electromagnetic Radiation

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Electromagnetic radiation is a type of energy that is emitted and absorbed by charged particles.

The Electromagnetic Spectrum

Electromagnetic radiation is classified by wavelength, which is inversely proportional to frequency (as per c=f*lambda, where the higher the wavelength, the lower the frequency and vice versa).

EM spectrum.png

The radiation with the greatest wavelength is radio waves, with a wavelength of approximately 1mm to several km. These can be further subdivided, with microwaves (SHF and EHF) being of the shortest wavelength, then UHF, VHF, HF, MF, LF, VLF, ULF, SLF, and ELF, where ELF has the longest wavelength. The differences between their uses lie in how far they can penetrate certain materials, whether the atmosphere blocks them, and how easy they are to produce. Generally, radio waves are produced by a transmitter and antenna. The antenna is excited by an alternating current and produces radio waves, with the wavelength determined by the frequency of the current.

Radio waves are used in radio communications, RADAR, MRI, radio astronomy, and many other functions.

Microwaves are usually generated by vacuum tubes, such as the magnetron in a microwave oven. They are used in communications, RADAR, microwave ovens (causing dielectric heating in food), wi-fi, and spectroscopy.

Terahertz radiation lies between microwaves and infrared. It ranges from 0.1mm to 1mm in wavelength, and can pass through many non-conducting materials. It is used in medical imaging, surveillance, spectroscopy, astronomy, and manufacturing (materials thickness testing). The record for wireless data transmission has been set by this kind of radiation, around 20 times faster than standard wi-fi (as of May 2012).

Infrared radiation has wavelengths of 750nm (a nanometre (nm) is 10^-9 metres) to that of terahertz radiation. Most thermal radiation emitted by objects at room temperature is in these wavelengths. Most radiation from the Sun comes in this form. It is emitted and absorbed by molecules when their rotational-vibrational movements change, making it very common indeed. It is used in night-vision technology, thermal imaging, heating, communications, weather forecasting, surveillance, military target acquisition, homing, tracking, astronomy, and spectroscopy. It can be dangerous, as strong IR radiation is an eye hazard. Some animals can see IR radiation.

Visible light has wavelengths of 380nm to 740nm. The radiation emitted by the Sun peaks in the visible spectrum, and atoms emit and absorb light at discrete energies. Molecules can spontaneously emit light, like in neon signs, flames and mercury vapour lamps, or be stimulated to do so, such as in lasers. Certain chemicals emit visible radiation (chemoluminescence) and some of these are used by organisms (bioluminescence). Phosphorecent materials can be made to emit visible radiation by being bombarded with subatomic particles, such as in a cathode-ray monitor. Light is used in communications, lasers, and for entertainment or advertising, among other uses.

Ultraviolet radiation is between 10nm and 400nm. The Sun emits UV light, and so do electric arcs and blacklights. A number of insect and bird species (and, apparently, reindeer) can see UV radiation, as well as certain humans. Ultraviolet is damaging to biological systems, particularly DNA, and causes sunburn, though it is important in producing vitamin D. The ozone layer blocks much of the Sun's ultraviolet light from reaching the Earth. UV light can also damage the eye.

Below ultraviolet light, EM radiation is ionising. Photons of ionising wavelengths can remove electrons from atoms without raising their temperature. This makes them hazardous.

X-rays are in the range of 0.01 to 10nm. In some countries, they are referred to as Röntgen radiation, after Wilhelm Röntgen, who is usually credited as its discoverer. X-rays of 0.1 to 10nm are referred to as 'soft' x-rays, and those from 0.01 to 0.1nm are 'hard' x-rays. Hard x-rays have greater penetrative ability. X-rays are used in medical imaging and radiotherapy, and are emitted by nuclear explosions. They are harmful in large doses.

Gamma-rays are the form of EM radiation with the shortest wavelength, with wavelengths of less than the diameter of an atom. They are produced in radioactive decay and gamma-ray bursts, among a few other sources. They are very penetrating, and dense, large-atomed elements such as lead provide the best defense against gamma-rays. Some wavelengths of gamma-rays are blocked by the atmosphere, but those that can get through are considered the most dangerous. They can cause radiation burns and radiation sickness.