Electromagnetic radiation

Electromagnetic (EM) radiation  has an electric and magnetic field component which oscillate in phase perpendicular to each other and to the direction of energy propagation [see diagram below].

The electric field and the magnetic field are tightly interlinked, in two senses. First, changes in either of these fields can cause ("induce") changes in the other, according to Maxwell's equations. Second according to Einstein's theory of special relativity, a magnetic force in one inertial frame of reference [An inertial frame of reference is one in which Newton's first law of motion is valid] may be an electric force in another, or vice-versa.

Depending on the circumstances, electromagnetic radiation may behave like a wave or as particles. As a wave, it is characterized by a velocity (the speed of light), wavelength and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hv, where E is the energy of the photon, h = 6.626 × 10^-34 J·s is Planck's constant, and v is the frequency of the wave.

Electromagnetic radiation is a ‘self-propagating wave’ which can be  propagated in a vacuum as well as in a material medium. Most other wave types cannot propagate through a vacuum and need a transmission medium to exist.
 
Waves - A wave is a disturbance that propagates through space and time, usually with transference of energy.  The wave produced by a transmitter is a sine wave.  To help understand this form of wave we might think of it loosely like the waves in the sea.  They are of a certain ‘height’ a certain ‘shape’ and travel forward with a certain speed or velocity.  Ripples on a pond also follow the same sort of pattern when you throw a stone in.

Waves have frequency and amplitude.  Their frequency is a measure of the number of occurrences of the wave oscillation per unit of time – so for example in our example of the sea wave, if we are standing at a certain point watching them then we might notice that there are 7 waves every minute – a frequency of 7 per minute.  The amplitude [as shown below in the graph] is the vertical distance between the axis of propagation and the peak of the curve – our sea waves might be 2 metres high, our pond wave only a couple of centimetres.

 

The frequency of an EM wave is its rate of oscillation and is measured in hertz, the International standard unit of frequency, where one hertz is equal to one oscillation per second.

Wavelengths - Waves also have wavelength -  a key property in transmission [see above]. Wavelength is the ‘distance’ between two identical points on the curve measured at a single point in time.  So to go back to our sea wave example, the waves may have been 5 metres apart at 14.30 on day XXX, the ripples on the pond only 10 cms apart. 

The wavelength is related to the frequency by the formula:

wavelength = wave speed / frequency.

Wavelength is therefore inversely proportional to frequency. Higher frequencies have shorter wavelengths. Lower frequencies have longer wavelengths. 

All EM radiation has the same speed [relative to the observer] – which in a vacuum is exactly 299,792,458 metres per second (about 186,282.397 miles per second). The speed of EM radiation depends upon the medium in which it is traveling and its frequency, thus some frequencies do not go this speed in air, for example, and some frequencies won’t go through some things at all but bounce off.  In a medium (other than a vacuum), velocity of propagation or refractive index are measures of the conductivity of a medium. Both of these are ratios of the speed in a medium to speed in a vacuum.

If now look at a wave from the point of view of time – see graph below, the term period is used.  The period is the time for a single oscillation and is the inverse of frequency.   Hence if the frequency of our sea waves is 7 per minute, their period is approximately 8.5 seconds.  The phase [not shown] determines the starting point on the sine wave.

Anything which emits EM radiation can also absorb EM radiation at the same wavelength (photon energy).

The Electromagnetic spectrum - The electromagnetic spectrum extends from below the frequencies used for modern radio (at the long-wavelength end) through gamma radiation (at the short-wavelength end), covering wavelengths from thousands of kilometres down to a fraction the size of an atom.  It's thought that the short wavelength limit is the vicinity of the Planck length, and the long wavelength limit is the size of the universe itself, although in principle the spectrum is infinite and continuous.
EM radiation is classified into types according to the frequency of the wave and wavelength, these types include (in order of increasing frequency and decreasing wavelength):

• radio waves - which have the longest wavelengths between 1m and  100,000 kms

• microwaves  - with wavelengths ranging from 1 mm to 1 m

• terahertz radiation – with wavelengths ranging between 1 millimetre (high-frequency edge of the microwave band) and 100 micrometres (long-wavelength edge of far-infrared light).

• infrared radiation - Infrared radiation has wavelengths between about 750 nanometres ( one millionth of a millimetre) and 1 mm. Humans at normal body temperature can radiate at a wavelength of 10 microns [one thousandth of a millimeter or one millionth of a meter].  To provide some idea of scale a DNA chromosome [a Chromosome is an organized structure of DNA and proteins that are found in cells] can be about 0.2 to 20 µm [one millionth of a meter].

• visible light - A small window of frequencies  is sensed by the eye of various organisms,  with variations of the limits of this narrow spectrum.  A typical human eye will respond to wavelengths in air from about 380 to 750  nanometres (one millionth of a millimetre).

• Ultraviolet radiation  - has wavelengths from 10 nanometres up to about 400 nanometres.  It thus overlaps slightly the range of visible light.  Ultraviolet light is classified into 3 broad groups A, B and C.  The Sun emits ultraviolet radiation in the UVA, UVB, and UVC bands, but because of absorption in the atmosphere's ozone layer 98.7% of the ultraviolet radiation that reaches the Earth's surface is UVA.   Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths, as such even if the EM radiation in this spectrum was safe, it would not be suited to transmission.

·  UVA, with a wavelength of between 300 and 400 nanometres, is considered the safest of the three spectra of UV light, although none of them are actually that safe for humans. UVA light is much lower in energy and does not cause sunburn but it is capable of causing damage to collagen fibers, so it does have the potential to accelerate skin aging and cause wrinkles. UVA can also destroy vitamin A in the skin.

·  UVA induces the production of vitamin D in the skin. Too much UVB radiation leads to direct DNA damages and sunburn. So UVB cannot be used for transmission.

·  UVA rays are the highest energy most dangerous type of ultraviolet light.

• X-rays - have a wavelength in the range of 10 to 0.01 nanometres. X-rays are a form of ionizing radiation and as such can be dangerous and would not be used for transmission.

• Gamma rays – which have the shortest wavelengths - below about 10 picometres - and the highest frequency and energy content 100 keV.  Due to their high energy content, gamma rays can cause serious damage when absorbed by living cells and as such would not be used for transmission.

[Laser light can in theory exist in any part of the spectrum.  It is coherent light radiation. Two waves are said to be coherent if they have a constant relative phase, which also implies that they have the same frequency.  The term "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. A typical laser emits light in a narrow, low-divergence beam, with a narrow wavelength spectrum ("monochromatic" light).]