Understanding the Electromagnetic Spectrum
Energy travels and appears in many different forms. The light energy that allows us to see is a form of electromagnetic radiation, however this is only a fraction of all the electromagnetic radiation that exists. While the different types of radiation along the electromagnetic spectrum may seem unrelated, they all share similar properties.
Unlike mechanical waves, which require a physical medium to travel through e.g. sound in solid, liquid or gas, electromagnetic waves can travel through a vacuum of space.
Electromagnetic radiation is produced when an atomic particle, such as an electron, is accelerated by an electric field, causing it to move. The created movement produces oscillating electric and magnetic fields, both of which are at right angles to each other, forming energy called a photon. Photons travel in waves through the vacuum of space at the speed of light (2.997 924 58 ×108 m/s).
Electromagnetic waves have specific properties which allow measurement and classification, and which explains the different nature of energy produced between the different segments. Figure 1 below shows an illustrated diagram of typical electromagnetic waves. The highest point of the wave is known as the crest and the lowest point of the wave is known as the trough. The distance between the crest and the central axis makes up the amplitude of the wave. The amplitude informs of the intensity or brightness of a wave in comparison with other waves.
Figure 1: Breakdown of an electromagnetic wave
Two important properties of electromagnetic waves are their wavelength and frequency. The wavelength (λ), measured in metres (m) is the horizontal distance between two consecutive crests (as shown in figure 1). The frequency (ν), measured in hertz (Hz), corresponds to the number of waves that form in one second. These two properties are inversely related. The higher the frequency, the smaller the wavelength, and vice versa.
The electromagnetic spectrum provides classification of the entire range of electromagnetic radiation according to their wavelengths, extending from the shortest gamma rays to the longest radio waves. The full electromagnetic spectrum is shown below in Figure 2. Non-electromagnetic radiation energy types are not seen on the electromagnetic spectrum.
Figure 2: The electromagnetic spectrum
As shown in Figure 2, there are seven main segments of the electromagnetic spectrum; gamma rays, X-rays, infrared rays, ultraviolet rays (UV), visible light, microwaves and radio waves. The range of electromagnetic radiation that affects erythropoietic protoporphyria (EPP) is located in the ultraviolet and visible regions, between wavelengths 320nm to 650nm, where after exposure to these wavelengths accumulated PPIX undergoes a chemical reaction resulting in swelling, severe and intolerable pain and scarring, a condition known as phototoxicity. For more information on EPP please click here.
The difference in frequency and wavelengths of the respective segments results in the differently produced properties, as described below:
Gamma rays have the shortest wavelengths, lower than 0.1nm, but produce the most energy, with frequencies above 1019 Hz. Gamma rays are ionising radiation and thus biologically hazardous, meaning that they can cause serious harm to human tissue following even brief exposure. These properties have allowed gamma rays to be utilised more broadly in the medical field; including killing cancer cells, usage as tracers and sterilisation of medical equipment.
X-rays are electromagnetic waves with wavelengths in the range of 0.01nm to 10nm and frequencies in the range of 3×1016 Hz to 3×1019 Hz. Similar to gamma rays, X-rays are also ionising radiation and can harm human tissue. X-rays are distinguished from gamma rays from the source of production. Gamma rays originate from the nucleus of a radionucleotide after radioactive decay whereas X-rays are produced when electrons strike a target, or when electrons are rearranged within an atom.
Very high doses of X-rays can cause radiation sickness, however, in short controlled doses they have been able to be utilised in medical imaging examinations, with the benefits of examinations far outweighing the risk of exposure.
Ultraviolet light comprises electromagnetic waves with wavelengths in the range of 10nm to 400nm and frequencies in the range of 1015 Hz to 1017 Hz. Most ultraviolet light is non-ionising, however higher wavelength UV light can be harmful and cause cancer.
UV light typically comes from sunlight and is subcategorised into three main regions; UVA (320-400nm), UVB (280–320 nm), and UVC (100–280nm). The majority of UVB and UVC are absorbed by the Earth’s ozone layer, thus consequently over 95% of UV radiation reaching Earth is UVA. The extreme higher ultraviolet region, approximate wavelength size between 10-200nm, is known as the ‘vacuum UV' region and is quite harmful and able to cause cancer (particularly in wavelength region between 10nm to 120nm). However, this region is strongly absorbed by the Earth’s atmospheric oxygen, thus blocking potential harmful radiation.
Although invisible to the human eye, we are familiar with the effects of UV light as it causes damage and sunburn to human skin, the photoprotective tanning response (activation of melanin synthesis) and, with prolonged exposure, can result in skin cancer. Conversely, exposure of human skin to UVB light triggers the production of vitamin D, which has an essential role in calcium absorption and bone health.
UV light has been replicated artificially for use in a number of different applications including medical therapeutics, tanning booths, lighting sources, and lasers.
Human eyes are only able to see a very small portion of the electromagnetic spectrum, visible light, which has wavelengths of between approximately 400nm to 700nm. Red light has the lowest frequency and longest wavelength while violet light has the highest frequency and shortest wavelength. As the human eye is unable to view any other segment, any images of other segments of the spectrum are actually in ‘false colour.’
Despite being non-ionising radiation, it has been shown to induce a number of reactions within humans, including skin erythema, the production of reactive oxygen species (ROS) resulting in direct and indirect DNA damage and photoaging, as well as the ability to activate skin pigmentation.
Infrared radiation comprises electromagnetic waves with wavelengths in the range of 700nm to 1mm and correspond to frequencies in the range 430 THz to 300 GHz. Infrared radiation is popularly known as “heat radiation” however electromagnetic waves of any frequency will heat surfaces that absorb them.
Infrared radiation is typically used to seek the temperature of objects and individuals and is widely used in the military; however, this technology has started to become cheaper to produce and is beginning to reach the mass public market for usage as infrared cameras on cars. Other uses include for heating, meteorology, spectroscopy, and astronomy.
Microwaves are electromagnetic waves with wavelengths in the range of 1mm to 25um and correspond to frequencies in the range 300 MHz to 300 GHz. The most common usage of microwaves is found in most household kitchens, the microwave used to re-heat food. However, microwaves are also utilised in telecommunication by mobile phones and also by circuits and radars.
Radio waves have the longest wavelengths, greater than 1mm, but produce the least energy and frequencies below 108 Hz. Radio waves are non-ionising radiation and have been utilised in a number of different communication and broadcasting areas including, e.g. radio communication, navigation systems, military communication, satellites, computer networks etc.
References & further reading
Australian Government, Australian Radiation Protection and Nuclear Safety Agency (ARPANSA), Understanding radiation, https://www.arpansa.gov.au/understanding-radiation/what-is-radiation/ionising-radiation/gamma-radiation accessed 14 May 2018
National Aeronautics and Space Administration, Science Mission Directorate. (2010). Introduction to the Electromagnetic Spectrum. Retrieved 14 May 2018, from NASA Science website: http://science.nasa.gov/ems/01_intro