Electromagnetic radiation principles
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Written by admin

Jan 24, 2020

January 24, 2020

The following blog is a compilation of exerpts from Open University text modules TM111 and TM129 and serves as a mean to better help me understand and remember certain physical and computing principles and ideas, and therefore all credit goes to the Open University authors of the text modules.

Electromagnetic radiation is a combination of an electric and a magnetic field. Radio waves, microwaves and light are all electromagnetic waves, although they differ substantially in what is called the frequency of the wave. The term ‘radio waves’ is sometines taken to refer generically to the whole of the radio and microwave bands. It is a generic term for electromagnetic radiation within a paticular range of frequencies.

Electromagnetic waves have the shape of a sine wave, i.e. they are sinusoidal. The graph of the electromagnetic wave function plots the electric or magnetic field strength against different points in space.

Wave lengths can also be measured at different points in time as it travels through a single point in space. The wave oscillates regularly and repeatedly with time around its average value. The wave plotted in time looks similar to the wave plotted in space.

The duration of one cycle of the wave plotted against time is known as the period of the sine wave. The frequency of a wave is the number of cycles it completes in a single second and the period is the duration of a cycle. The shorther the period, the more cycles completed in one second, so the higher the frequency. Mathematically this is expressed as folows, frequency = 1/ period (and period = 1/frequency). Alternatively, the speed of a wave = wavelength x frequency.

The unit for measuring frequency is the hertz (Hz), i.e. a wave that completes one cycle every second has a frequency of 1 Hz. A wave that completes 1000 cycles every second has a frequency of 1000 Hz or 1 kHz (kilohertz), and therefore a period of 1 ms. Higher frequencies can be expressed in terms of MHz (megahertz: 1000 000 cycles per second) or GHz (gigahertz: 1000 000 000 cylcles per second).

As such, the frequency, the period and the wavelength of a sine wave are all related, and in particular, electromagnetic waves are described sometimes in terms of their frequency and sometime in terms of their wavelength.

 

Sunlight contains electromagnetic waves with a range of different frequencies; when light is split up into the colours of the rainbow, the different colours correspond to different frequencies. The two ends of the rainbow spectrum correspond to the two extremes of visible radiation, with violet light having a higher frequency than red light. However the sun also emits radiation at other frequencies that are not visible. Radiation with a frequency slightly higher than that of visible violet light – off the ‘top end’ of the visible spectrum – is known as ultraviolet (UV) ligh which in excess can damage human skin and eyes. Off the other end of the visible spectrum, infrared radiation has a frequency slighly lower than that of visible red light. Infrared radiation warms the skin (as does visible light), but is not harmful. Some animals can see outside the ‘visible’ spectrum. For example, some fish can see infrared and penguins can see ultraviolet light.

The full range of electromagnetic radiation is described as the electromagnetic spectrum. The electromagnetic spectrum is one of the most common carriers of sensory information. Although only one small part of this spectrum is visible to humans, we can design sensors to detect others. e.g. X-ray machanies and devices that use infrared light.

By looking for reflections of high-frequency electromagnetic radio pulses that are invisible to the human eye, we can use radar to build up a picture of what is around us up to several miles distant. For example, radar can be used to make weather maps.

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