Light: Life’s window to the world

Electromagnetic Energy is not only the primary source (or secondary source after gravity…) of energy on Earth, ‘light,’ the smaller fraction of the electromagnetic spectrum, is also an important source of information for many forms of organisms. The different properties of light, such as intensity, duration, polarisation, and spectral composition, can all be used as sources of information.

In all, light sensing is connected to movement in some way so that, once signalled, the creature can respond. [1]

Life on Earth has developed three principle forms of light detectors, known as photoreceptors: flavin-based blue-light photoreceptors (i.e., cryptochromes), retinal-based green-light (such as rhodospin), and linear tetrapyrrole-based red-light sensors (i.e., phytochromes in green plants).

Animals

In animals the detection of light leads to vision. In its simplest form, light-detecting cells in worms for instance, are scattered across the skin (although concentrated near the head), helping them detect warmth and sunlight, as too much of both will dry them out, and UV light which kills the delicate nerve-endings, causes paralysis.

The honeybee: like us, is trichromatic - it has three different photoreceptors in the eye, that builds up its' view of the world. Bees however, do not have red, blue, and green receptors (like us), instead they have ultraviolet, blue and green (they can’t see red) receptors. This ultraviolet vision allows them to see the “bulls-eye” markings, that many plants (i.e., primroses and pansies) have on their flowers, guiding bees like runway lights to the nectar. Bees, as well as two fixed compound eyes on either side of the head, have three smaller eyes, called ocelli on the top of the head. Ocelli sense light intensity, but not images; and are believed by scientists to be involved in navigation - and the fact there are three, they may even provide a triangulation function.

Other more complex visual systems, such as compound eyes can be found in insects and crustaceans for example, that consist of several thousand light detectors called ommatidia. These are particularly good at sensing movement, and a broad range of colours (i.e., the Mantis Shrimp, which possesses ‘hyperspectral’ colour vision).

Single-lens eyes, such as our own, have also evolved into various forms in fish, birds, mammals, and spiders for instance. Some snakes (such as pit vipers) also have the ability to sense infrared thermal radiation (IR) between 5 and 30 μm, through sensors located in their noses - ‘smelling’ heat - giving them the ability to not only locate prey, but also vulnerable body parts in the dark. However. this is not strictly a photoreceptor, it is more a temperature sensor.

Plants

Plants, as well as being able to use light as an energy source, are also able to detect light (due to light’s importance for energy), which can result in changes in growth and morphology (form) - known as photomorphogenesis. Some plant seeds, such as many lettuce varieties, need light to germinate (positive photoblastic), or when detecting a lack of light will continue to lay dormant. Photoperiodism is a physical response, such as flowering, which is made by red-light detection during a 24 hour period. In this case, plants only start to produce flowers when they detect a critical night-length, which signals the appropriate time of the year to bloom.

Phototropism is a physical response, where blue-light detectors, help direct the plant to grow shoots towards the sun; much like heliotropism, which describes the ability of some plants (such as sunflowers) to track (and turn with) the Sun’s motion on a daily basis - and some other plants ability to raise and lower their leaves on a daily basis (i.e., leaf heliotropism of many legumes). Light detectors in plants can also determine the quality of the light - helping plants to avoid growing or even germinating (like oaks) in the shade. Finally, chloroplasts - the organelles, mostly located within plant leaf cells, where photosynthesis takes place - are also able to move towards or away from light within the cell - again optimising the amount of light that the plant can catch. [2]

Fungi

Most fungi, are able to sense and react to light, as many have all three of the major classes of photoreceptors (exceptions may include some forms of yeast - single cell fungi, for instance). Some fungi, such as Cyathus stercoreus require light to initiate fruiting body development. Other fungi, such as sporangiosphores, grow towards the light (not bend towards light like plants) - phototropism, and the fruiting body of basidiomycetes depends on light at different development stages - and in:

…ascomycetes, light also has a strong impact in morphological and physiological processes, including the regulation of conidial germination, hyphal branching, sexual and asexual development, and secondary metabolism. [3]

Bacteria

Photosynthetic bacteria, such as Cyanobacteria (among the world’s most important oxygen producers) are able to detect different wavelengths, as they migrate up and down water columns in the worlds oceans - at the water surface light still has a wide spectrum, but at depth, blue light dominates.

Image Sourcehttps://doi.org/10.7554/eLife.12620.011

Cyanobacteria, and two slightly larger organisms, protists, Erythrodinium, and Neodinium, are able to use their entire bodies, as a microscopic light-sensitive focusing lens device (much like a camera lens), a kind of single-cell eye. Using the information to move towards or away from light (phototaxis), through patches of motor proteins, forming…

…on the side of the cell facing the light source. Pili [hair like appendages on the surface of many bacteria] are extended and retracted at this side of the cell, which therefore moves towards the light. [4]

Light as Communication

Many different species within all the kingdoms of life, are able to manipulate light as a form of communication. Some plants, for instance, have the ability to create a 'bluehalo' of light (known as structural colours) around their flowers to attract pollinating bees [5]; or, have the ability to use specific pigments (pigments absorb and/or reflect light in certain wave-lengths), such as those that reflect UV, which are visible to insects (but not to us), and act as landing lights, guiding the insects towards the flowers nectar (and therefore, pollen). Bioluminescence, is the production of light through a chemical reaction, in an organisms body [6]. Fireflies, and many marine animals, from algae to jellyfish and crustaceans (and symbiotic bacteria living in some of these organisms), are able to control the chemical reaction, which is used for attracting mates, feeding and protection.

Humanity also uses light for communication. Beacons of fire (sometimes positioned in a network), have been used in different parts of the world for centuries, as a form of communication across distances upto 100 km. In the 3rd century BC, ancient greeks were transmitting and receiving different messages by varying the combination and position of lit touches [7]. This form of communication evolved to the more modern, Morse code, invented by Samuel Morse in 1832 (which can also be transmitted via radio waves and electrical pulses), which is able to transmit the entire alphabet and numbers 0 to 9 by varying the duration of the transmission: generating a series of dots and dashes (a dash lasts three times longer than a dot).

Modern technology also include the transmission of data via optical fibres, and the use of laser beams to read information from pitted patterns in a spiral track on CD's.

A more recent technology invention, known as Li-Fi, uses LED lights, much like Morse code, to transmit data (and position) through very high flickering rates (invisible by the human eye), which are able transmit the equivalent to the '0''s and '1's of digital code.[8]

References

[1] Margulis, Lynn., and Sagan, Dorion. (1995) ‘What is Life?’ University of California Press, Berkeley and Los Angeles, California.

[2] Suetsugu N, Higa T, Gotoh E, Wada M (June 16, 2016) 'Light-Induced Movements of Chloroplasts and Nuclei Are Regulated in Both Cp-Actin-Filament-Dependent and -Independent Manners in Arabidopsis thaliana' PLOS ONE 11(6): e0157429. https://doi.org/10.1371/journal.pone.0157429

[3] Fischer, Reinhard et al. (2016) ’The Complexity of Fungal Vision.’ Microbiology Spectrum, American Society for Microbiology Press.

[4] Schuergers, Nils et al. (Feb 9, 2016) ’Cyanobacteria use micro-optics to sense light direction.’ eLife 2016;5:e12620 DOI: 10.7554/eLife.12620

[5] Physics World, Biophysics (retrieved April 2018) 'A flower's nano-powers.' https://physicsworld.com/a/a-flowers-nano-powers/ 

[6] Smithsonian, Ocean - Find your Blue. Fish. (retrieved April 2018) 'Bioluminescence.' https://ocean.si.edu/ocean-life/fish/bioluminescence

[7] Kotsanas Museum of Ancient Greek Technology (retrieved October 3rd 2017) http://kotsanas.com/gb/exh.php?exhibit=1201001

[8] Wikipedia, (retrieved April 2018) 'Li-Fi' https://en.wikipedia.org/wiki/Li-Fi

Electromagnetic spectrum and emittance

EM Spectrum

The Sun emits electromagnetic radiation in many different wave lengths. The actual EM radiated by the Sun, is known as the solar spectrum, and is made up of some ultraviolet (A, B, and C), but mostly ‘visible light’, and infrared (A, B, and C) - as heat. It extends from around 290 nm to more than 3200 nm. Some stronger rays are emitted by the Sun (such as X-Rays), as mentioned in previous sections, but these do not make it through the Earth’s lower atmosphere. Small amounts of radio waves are also given off by the Sun and other stars. The energy spectrum given off by a star depends upon its size and temperature.

The ‘visible spectrum’ - that part of the spectrum that we can detect with our eyes is only a small part of the entire spectrum of electromagnetic waves. To the left of visible light are wavelengths that increase (higher) in frequency - short waves, and to the right are waves that decrease (lower) in frequency - and therefore long waves. As stated above, all forms of electromagnetic energy travel at the speed of light, but the amount of energy of a photon differs across the spectrum - from left to right, from more powerful gamma rays (i.e., 106 electron volts), X rays and Ultraviolet (and more harmful to life) on the left, to less powerful infra-red, and radio waves (less harmful for life - i.e., 10-8 election volts) to the right.

As is shown in following web pages, the Earth’s atmosphere filters out nearly all the most dangerous forms of electromagnetic waves.

Emittance

All objects have charged particles moving around within them and interactions occurring - which changes their velocities, and elements vibrating and rotating, therefore emits (and absorbs) EM radiation (unless the object’s temperature is OºKelvin - absolute zero). Therefore all objects, such as ourselves, the air, walls, and an apple pie, emit (and absorb) electromagnetic energy, and the Sun can be said to produce EM radiation, as it emits, but absorbs virtually no EM radiation.

The Importance of Temperature (Thermal Radiation)

In understanding, and being able to calculate objects emittance of EM radiation, scientists developed experimental test rigs using empty boxes, internally covered in black soot, for instance called a blackbody, creating an idealised model, to compare against experimental results.

What scientists (such as Planck and others) eventually found with these experiments, was that the amount (emission rate or intensity), and type (spectral distribution) of EM radiation emitted and absorbed, depends on temperature. They found that:

Hot objects emit energy at a higher rate (greater intensity), cold objects far less.

And that most objects emit EM radiation at many different wavelengths - although there is usually one particular wavelength that is emitted the most (known as lambda (λ) max) - and:

Hotter objects emit a greater range of wavelengths, and as temperature goes up λmax shifts to the left.

To understand what this means, and the terminology described above, the first picture to the right shows a light bulb (which is connected to an electrical circuit for power…).

Incandescent lamps glow bright as electric current flows through a filament. The filament is a conductor, which allows electricity to flow through it. However, filaments are made short and thin, which increases the resistance, and so the flowing current basically bumps into the atoms in the filament a lot, as they ‘try’ to travel through it, increasing the temperature. As already stated above, hot objects emit (more) EM radiation, and so the objective of the incandescent bulb is actually to get the filament really hot - although this means a lot of energy is wasted as heat (IR radiation).

The first image ‘Off’ (to the left) shows that there is a measure of EM radiation emitting from the bulb even though the light is switched off. As the atoms are being accelerated and/or decelerated in the bulb, like in any object, they are emitting some EM radiation to the room, however, as the light bulb is off and cold, it is only giving off very, very small amounts of long-wave infrared into the room, that is too cold to feel as heat.

The next image ‘Slightly On’ shows that when the light bulb is slightly on, the emitted radiation shifts to the left - or as scientists say - shifts to the blue as it starts to go towards the blue spectrum of visible light. Here the bulb does emit small amounts of visible light in the red spectrum (as well as more IR), and so part of the EM radiation is now visible as low intensity red light, and perhaps a small amount of heat can be felt.

The last image ‘Fully On’ shows, again, how the curve shifts further to the left, now that the light is fully on, and the mix of the light spectrum now being emitted gives the lamp a “warm” white glow. The amount of emitted IR radiation has also increased again, and so the bulb is hot to touch.

This basic light bulb example confirms that hot objects emit energy at a higher rate (greater intensity), cold objects far less. It also shows that most objects emit EM radiation at many different wavelengths - although there is usually one particular wavelength that is emitted the most (known as lambda (λ) max). And it shows that hotter objects emit a greater range of wavelengths, and as temperature goes up λmax shifts to the left.

The experiment also shows that the intensity spectrum is a bell curve - there is a max intensity, and after which, short-wave length emission drops to zero. This was not obvious, at least in theory to scientists for a long time - although experiments showed bell curves as observed results. Their calculations showed that the curve should keep increasing from the right: as temperature increased, intensity would increase to infinity as wave length shortened (known a the UV catastrophe). It took Plank, to suggest that light was not a continuous wave, and instead ‘packets’ (as discussed in part 2) to make the maths work, and later Einstein to confirm that light was actually packets (also particles) in the photo-electric effect experiment that made the calculations (which are relatively complex) show what was observed in experiments.

Black Bodies

Black body radiation is an ideal model (black bodies do not really exist) for the EM radiation given off by an ‘ideal’ (or ‘perfect’) radiator. In general, an object can absorb the same EM radiation as it can emit, however, as a perfect absorber, it will reflect no light (as all is absorbed), and therefore will look black (hence ‘black body’). Black bodies also emit EM radiation at all wavelengths.

Black body radiation calculations purely consider temperature (which must be constant - in thermal equilibrium), and it ignores size, and material composition (discussed below). As a model it can be used to compare against what is observed, and although it is a theoretical model, surprisingly, the Sun is very close to a black body concerning absorption, as any light that hits it’s gas/plasma surface is scattered and totally absorbed. However, it is not a perfect black body, concerning emittance: The sun does emits light across the entire EM radiation spectrum, but it doesn’t follow exactly the ideal model, due to different dynamics happening inside the plasma, but it can be considered a black body for most calculations. The first of the images to the right shows the ideal black body radiation, with all the properties discussed with the light-bulb example above. And the second image to the right shows that actual emittance of the Sun (orange line filled with yellow. As can be seen from this graph, the ideal black body radiation for an object of this temperature (5778 Kelvin) follows very closely to the measured emittance.

The Importance of Material Type

As mentioned above, most objects are not black bodies, in that others factors, particularly what the object is made of is also very important. For instance, when iron is heated to the same temperature as glass, it emits and absorbs a different amount and type of EM radiation - it has a different emission spectrum.

Understanding how EM radiation (energy) interacts with materials (matter), at the atomics level and at the molecular level will be discussed further in some of the further web pages; as the EM radiation begins to penetrate the Earth’s atmosphere and interact with the Earths surface.

Waves, particles, and duality

Waves

There are two main types of waves, mechanical waves and electromagnetic waves: and these are the main ways that energy moves from point A to point B: in other words, all waves transport energy as they travel.

Mechanical waves need a medium for energy to be transferred through - they can not travel through a vacuum. Mechanical waves, include ocean waves, seismic waves, or sound waves for instance - i.e., sound needs water or air to vibrate through. The fact that sound cannot travel through space, is probably a good thing concerning the Sun, as the noise given off by the Sun has been estimated to be around 100db, and if it could travel through space to Earth, it would be as loud as a mechanical hammer - all day-long, every-day… Mechanical waves can be both longitudinal (which is basically a compression or compaction being transmitted through a medium) or transverse waves.

Electromagnetic waves, such as visible light, do not need a medium - they can travel through the vacuum of space, but they can and do travel through mediums, and are effected by mediums - such as water, or air for instance - where they can be slowed-down, absorbed, reflected or refracted for instance. Electromagnetic waves are only transverse waves, and therefore, can also be polarised.

Although these forms of energy are different, they can all be represented as waves, as they all exhibit wavelike properties.

A wave can be defined as a disturbance propagating through space - which usually transfers energy. A single periodic wave, can be visualised in the image to the right.

A few properties that characterise a wave: Hertz (Hz), is the number of full wavelengths (λ - in meters) that pass a particular point (Point A in the image to the right) every second (s). It holds that the shorter the wavelength, the higher the frequency (ν - in Hertz) - they are inversely proportional - and as all forms of electromagnetic radiation travel at the speed of light (c: 3 x 108 m/sec (or 300 million metres per second) - the relationship is described in the formula c =  λν (same as λ x 1/T or λ x f).

Period (T) is the number of seconds per oscillation, and as frequency is the number of oscillations per second; frequency (f) = 1/T.

The second image to the right shows two waves with the same wavelengths, but different amplitudes (heights). With mechanical waves, amplitude normally means more energy (larger oceans waves carry more energy than smaller ones), however with electromagnetic waves, it describes more brightness, but not more energy, as will become clearer shortly.

Light Intensity (brightness) reduces the further away from a light source (just like gravity), the lit object(s) are from it. As shown in the image to the right, there is an inverse relationship to the square of the distance (inverse-square-law). Light must spread out over a larger surface area from the point source, over increasing distances, resulting in the reduction of brightness over a distance. For instance, the light intensity from the Sun on Mercury is estimated to be 9126 Watts/m2 (on average), compared to the greater distance to Earth, which receives ‘only’ 1367 Watts/m2. And so the threefold increase in distance, between these planets and the Sun, results in an approximate ninefold decrease in light intensity on Earth.

Light also redshifts if the light emitting object is moving away from the lit object (or viewer). Which is how scientists have proven that the universe is expanding.

The fourth image to the right, illustrates two waves with the same amplitudes (A), but with different wavelengths (λ). As all electromagnetic waves travel at the same speed – the speed of light (in a vacuum) – the number of waves (per second) that travel past point A depends on the wave length. This number is known as frequency (Hz), and so the shorter the EM wavelength the higher the frequency, and the higher the energy; and inversely, the longer the wavelength the lower the energy.

Electromagnetic radiation, can therefore, be described by its wavelength, amplitude (brightness), frequency, and period. But what is actually waving up and down?

These wave images, are not attempting to show photons flowing through space on a wave. Instead, the wave, is the oscillation of the value of the electric field (and magnetic fields as we will be showed later), as the photon travels through space at the speed of light.

Particles

Photons

Physicist Max Planck found that EM Radiation is not continuous - like the line in the wave image shown previously, instead energy is transferred in individual “packets” - or particles (of the size hv, ‘h’ being the Planck constant), oscillating as a wave. This was the discovery that light is both wavelike and particle-like - depending on the scale it is viewed at.

This “packet” became known as a ‘photon,’ and as described in the previous section on the transfer of energy through the Sun, photons interact with atoms and molecules, by being absorbed or emitted, or breaking molecules, or changing the energy properties of atoms and their electrons. Just as atoms can absorb photons, by energising outer electrons to a higher orbit; atoms (or molecules) that loose energy, also emit photons. The energy lost or gained by the atom or molecule, is directly proportional to the frequency of the photon, shown in Planck’s famous equation: E = hv. ‘E’ being the energy of the photon emitted or absorbed, and h being Planck’s constant, given in Joules (J).

Energy is microscopically deposited in discrete amounts - on a macroscopic scale it looks like a smooth continuous wave, but at the view of an electron, light deposits discrete - specific size chunks: which means they are quantised - they have clear quantities.

Photoelectric Effect

With the understanding that atoms absorb and emit energy only as discreet quantities (quanta), it was also proven with the photoelectric effect, that EM Radiation only travelled in quanta too; and so quantum physics was born.

An experiment first made by Hertz in 1887, showed that blue light could eject an electron (once ejected, is called a photoelectron) from a material (in this case potassium); but, red light couldn’t, even if the intensity (the amplitude) hitting the material was increased, and for long periods of time.

This went counter to classic wave (mechanical) theory  that understood that the ‘bigger’ (the higher amplitude of) the wave, irrelevant of its’ frequency (wavelength), it should heat up an electron until it was eventually freed from the metals surface. But the opposite was happening.

Einstein proposed correctly that, in fact, EM Radiation was actually travelling in quanta (photons), and not in continuous waves. And so, the photons have, like the wave, a specific frequency (wave length), but as a short ‘pulse’ or ‘packet’ their amplitude is better thought about in the numbers of photons; for instance, increasing the amplitude, increasing the number of photons travelling through space, it doesn’t generate (at least visually) a higher single wave. And so, unlike mechanical waves, EM waves do not increase in height with increased amplitude, and so a weak photon will not be able to knock an electron out of a metal, until the mimimim frequency is reached (threshold frequency), irrelevant of how many have been directed at it. And when the minimum threshold is met, electrons will be released, and an increase in intensity (more photons) will now release more electrons. This proposition, that was built on the work by Planck, was finally verified in experiments by Robert Millikan in 1916.

A standard 100-Watt light-bulb, for example, emits around 1020 photons per second.

Wave-Particle Duality

The Double Slit Experiment

One famous experiment that shows how light is (also) a wave, is Young’s Double Slit Experiment.

The first image to the right showed what the experiment would (should) show if light was indeed purely a particle (the orange dots): those particles that travel through the slits would predominately group very close to each other on the light detector screen inline with the slits.

The second image shows a light source on the left, which in modern experiments could be a laser, The laser is shone at a screen, which again, has two very small slits in it. The two holes are really close together - about the same distance apart as the wavelength of the laser light (making it possible for one single light ‘beam’ to be shone through both holes, to guarantee they are in phase). The orange arrows represent the light as waves (photons), and the larger orange curves represent the peaks of the photon waves, from a imagined top view. Beyond the light blocking screen, is the cavity, with a light detector at the back.

The second image shows how, the light that does goes through the slits, diffracts (bends), and spreads out, splitting into two new waves. The two new waves interfere with each other. As can be seen in the third image to the right: when a wave peak meets another wave peak, it creates a wave with a greater amplitude (more brightness) - known as a constructive wave; and when a wave peak meets a wave trough, they effectively cancel each other out (darkness) - known as a destructive wave. And so when the wave reaches the light detector screen, there is an unexpected wave pattern (known as an interference pattern) of particles, with the greatest intensity in the middle where there is the most constructive waves crossing prior to reaching the light detector screen, and effectively dark patches where the destructive waves have cancelled out the light.

More modern experiments have shown that there are even stranger things going on, like when a single photon (or electron) is fired at a time; the overall pattern after time on the light detector screen looks like the original interference pattern… is the electron predicting the future interference - or is it splitting in two and interfering with itself - or?

Electromagnetic radiation, an introduction

The energy and heat that reaches the Earth ultimately comes from the Sun, with vastly smaller amounts from other astrophysical sources (i.e., Pulsars and Quasars, and radio waves emanating from Jupiter).

The energy and heat released from the Sun comes in the form of electromagnetic (or EM) radiation, propagating outwards in all directions, and from the more sporadic solar winds.

An Overview

Electromagnetic (EM) energy, is a form of radiation, as it travels out as rays in straight lines, and is ‘electro’ ‘magnetic’ - a combination of both electric and magnetic - as it distinctly consists of two harmonic waves oscillating (‘swinging’ back and forth) perpendicular (90º) to each other - electric-waves, and magnetic-waves.

All EM waves travel at the fastest known speed in the universe - the universal speed limit: (c) 299,792,458 meters per second - known as the speed of light, as light also happens to travel at this speed limit.

The energy within EM radiation is held and transported equally in the perpendicular electric and magnetic fields propagating out into space.

This energy is measured in its irradiance or intensity. The irradiance energy from the Sun that reaches the top of Earth’s atmosphere is about 1350 Watts/m2, known as the solar constant. (Around half of this energy actually reaches the Earth’s surface).

EM energy also transports momentum (although it has no mass), which means that it is also able to exert a very tiny mechanical force on other objects: known as radiation pressure. It is this radiation pressure from photons escaping the Sun’s core, combined with thermal pressure that balances the compressive pressures from Gravity; and could also be harnessed to move a solar sail in space. This effect is virtually non-existent on Earth, however, so is not discussed any further in the main text below.

EM radiation can be understood as a wave of propagating electric and magnetic fields, and also as distinct massless energy ‘packets’ (known as photons).

EM radiation, therefore, is also understood to be particles as well as waves: known as the wave-particle duality. The energy in this packets (photons), for visible light, is very small: around 4 x 10-9 Joules - or between 1.8 electron volts (eV) for red light to 3 eV for blue light. It is important to note here that all atomic particles, such as electrons, have particle-like and wave-like aspects to their natures - not just EM radiation.

To understand this range of interrelated phenomena in more depth it is easier to first look at some of the basic concepts: electrostatics, wave-particle duality, and then magnetism; to understand them first as separate phenomena, before putting them all back together again, as part of a integrated emergent phenomena, as they are now recognised to be.

Electric Charge

There are three types of electric charge: positive, negative, and neutral. Protons have a positive charge, electrons have a negative charge, and neutrons have a neutral charge - so no charge.

Atoms and molecules that have an equal numbers of protons and electrons also have a neutral charge. Those atoms or molecules that have more electrons than protons have a net negative charge (and are known as anions), and those atoms or molecules that have less electrons than protons have a net positive charge (and are known as cations); both negatively or positively charged atoms or molecules are collectively known as ions.

Particles with the same charge repel each other, opposite charges attract, and neutral charges have no repulsion or attraction.

Even though electrons have a much smaller mass than a proton, they have the same level of charge. A proton has a charge of +e, and a electron has a charge of -e. And all electric charge is quantised - it comes in discreet units - small distinct packets of energy (meaning that half charges are not possible).

When a particle enters into the electric field (which is explained below) of a charged particle, it experiences a force - either attracting it or repelling it. This is the physical property of an electric charge. Electric charge is measured in coulomb (C) in engineering, and as elementary charge (e) in chemistry. The electric charge of a particle reduces across a distance - just like gravity; and can be calculated using Coulomb’s law: the force is proportional to the product (sum) of the charges, and inversely proportional to the square of the distance between them.

Coulombs law also shows that the electric force increases as the charge increase, and it can also be used (by chemists) to predict ionisation energy: the energy required to create an ion; in other words, the amount of energy needed for an atom or molecule, to loose an electron. As Coulomb’s Law states, that the force reduces over distance, this law shows how electrons close to the nucleus need a lot of energy to separate them from the nucleus (and those further away, less energy), and that the more protons in the nucleus (the greater the positive charge), the greater the energy required to pull electrons away from the nucleus.

Electric Current

When an electric charge is in motion it is known as electric current, with the intensity  (amplitude) usually measured in amperes.

Examples of electric current in nature can be observed in lighting, static electric discharge (i.e., by rubbing a ballon on ones head), and the solar winds effect on the Earth’s Ionosphere (creating the polar auroras).

Current can consist of any moving charged particle, such as an electron flowing through a conductive material (such as a metal), or a photon or electron through the vacuum of space, or ions (charged atoms) flowing through an electrolyte.

Creating a electric current creates a magnetic field.

This is known as Ampère's law. To the right, the image basically shows a green cable with a current (I) moving from a positive charge to a negative charge, with the generated circular magnetic field (B) produced around it, perpendicular to the current.

The relationship between electrostatics with magnetism in electric current, showed scientists that they are related phenomena, and more recently, that they are part of the same phenomena.

More about magnetism is discussed below, and in the page about the magnetosphere, however, it can be said here, that the early understanding of the relationship led to the invention of electromagnets - a technology that is used to run motors, generators, loudspeakers, and MRI machines for instance.

Electric Fields

All positively or negatively charged particles produce, and are influenced by electric fields.

An electric field is a measurable effect generated by any charged particle.

An electric field can be illustrated as a vector field (these do not really exist - it is a form of visualisation): that is to a say - a field that permeates out into space at all times, like spokes in a bicycle wheel, in all directions from the centre of a particle, to an infinite distance; and, like gravity, the  effect gets weaker the further the distance is from the particle centre.

The orange ‘equipotential lines’ represent this reduction in measurable effect over distance (closer circles have a greater potential force than circles spaced further apart).

An electric field can be calculated (and visualised) by placing a positive point test charge (which is a very small positive charge) in various places around the charged particle, which shows its’ electric field direction and magnitude.

The arrows indicate weather the electric field is repelling or attracting - outwards for positively charged particles, and inwards for negatively charged particles. For instance, when a positively charged particle interacts with a very small positive point test charge it will repel it (therefore outward pointing arrows), and when a negatively charged particle interacts with a very small positive point test charge it will attract it (therefore inward pointing arrows).

These vector arrows should not to be confused with an electric force, however, as a charged particle can only cause an electric force on (and with) another charged particle; a particle on it’s own in space has an electric field (and the vector arrows showing how it will effect - and be effected by - a positive test charge), but it does not create a force by itself.

Positively charged field lines, never cross, and always travel from positive to negative charge. As can be seen in the third image scenario, a positive charge (in this case a proton) has been attracted to a nearby negative charge (in this case an electron), and the field lines that were previously going straight outwards or inwards like wheel spokes have come and bowed and joined together; and the equipotential lines have been ‘squeezed’ in the middle and stretched on the outside. This third image becomes an electric field of a electric dipole. The fourth image shows the electric field lines for two repelling protons with the same positive charge, which, if there are no negatively charged objects around, will flow outwards for infinity, ‘in search’ for a negative charge.

The electric field combines with the magnetic field to form the electromagnetic field (this will be explained at the end of this section).

Magnetism - A Short Overview

So far, it has been illustrated that EM radiation is a propagating electric field that travels like a wave and a particle through the vacuum of space at the speed of light. However, at the beginning of this section, it was also highlighted that there are actually two waves propagating through space, together - and this second wave, not yet discussed, is the magnetic field. Magnetism is covered in more depth in the next webpage on the magnetosphere, so this section on magnetism will be brief.

Magnetic Fields

Magnets, instead of the ‘positive’ and ‘negative’ charges (monopoles) for particles, magnets have north and south dipoles. However, just like charges there is a repulsion between similar poles (like charges) and attraction between opposite poles (opposite charges). And this force also reduces over distance, and can also be calculated using Coulombs Law. Compasses can be used like the ‘test charges’ in electric fields, to understand magnetic fields, which again, look very similar (see image to the right).

The main difference in the two phenomena is that a proton (positive charge) an electron (negative charge), for example are individual elements - they are monopoles, however, it is impossible to split a magnetic into one individual south and north monopoles - and so magnets are always dipoles.

The first clear proof that both electrostatics and magnetics where part of the same phenomena was when scientists (such as Hans Christian Ørsted) found that by moving charge (making a current), a magnet would be deflected. And so: a moving electric field (a current) generates a magnetic field.

Therefore, a ring of compasses placed around a wire with a current flowing through it, made the magnets collectively point in a circle around the wire (see image in electric current section): In other words the magnetic field is perpendicular to the current, and therefore the electric field. The greater the charge, the greater the magnitude of the magnetic field, and the direction of the current determines the direction of the magnetic field.

When an electric field is moving a magnetic field is created (discussed above), and the scientist Michael Faraday proved that the opposite was also true: a moving magnetic field can generate a charge. This is called electromagnetic induction, and will be discussed more in the next page on magnetism.

Bringing it all Together

Therefore, it is currently understood that the energy that is released from the fusion reactions in the Sun, and is transported, in straight lines across space, is a combination of oscillating electric fields and magnetic fields, perpendicular to each other, in discreet ‘ packets (protons); and in a spectrum of wavelengths (discussed in the next section).

The journey of energy out of the Sun

This section goes into more detail on some of the main zones (differentiated by the main form of energy transfer taking place), between the core and the Sun's outer surface; describing the main reactions, occurring as energy slowly travels outwards, before finally being released into outer-space.

The Radiation Zone

Within the Radiation Zone, the Positrons quickly encounter high attraction collisions with the many free electrons (it’s antiparticle with a negative charge), annihilating both, and giving off two or more Gamma Ray Photons in the process. Unlike the Positrons, however, all the expelled Gamma Photons are not destroyed, instead their energy is radiated (transferred) through the absorptionemission and scattering of photons in the Radiation Zone Plasma.

Magnetic Dynamo

The Radiative Zone together with the Core rotates faster than the surrounding Zones (this is possible as the Sun is made up of plasma & gas, and is not a solid). Scientists believe this speed differential creates a very large shear, at the area of contact between these two layers, generating a huge Magnetic Dynamo – the dipolar magnetic field for the Sun (which flips its' North and South on a 11 year cycle). This area of immense shear is known as the Tachocline.

Convection Zone

After the Tachocline transition zone is the Convective Zone. Here, the visible light Photons' energy is sufficiently low, so that they can be absorbed by the solar plasma in the subsequent Convective Zone. They are not directly released, which makes the solar plasma atoms and ions (known as Granules) extremely hot. This heat makes the Granules rise to the surface – to the Photosphere – where they release their excess energy (again as Photons) creating a bubbling visible effect on the surface of the Photosphere, known as Granulation. As the energy is released, the Granules cool and then subsequently sink back down to the base of the Convection Zone, to repeat the process again. This process has been compared to a giant lava lamp.

Photosphere

As described below, the Sun has other layers above the Photosphere, however, it can be classed as the outer shell, as it is the first (lowest/deepest) surface of the Sun that is perceived to emit light. Below this layer, the plasma is opaque.

Atmosphere: Chromosphere, Transition Region, Corona, and Heliosphere

The atmosphere is composed of four distinct parts. The first, the Chromosphere, which is the 'coloured flash' that is visible at the start and end of a total solar eclipse. It is usually only visible during an eclipse, as it is not very dense, and is out-shone by the photosphere below. Many complex and dynamic phenomena, such as coronal mass ejections and coronal loops, discussed below, can be observed in the Chromosphere.

The second part of the Sun's Atmosphere, is the Solar Transition Zone. It is visible with ultraviolet sensitive telescopes, and marks the differences between the Chromosphere below and the Corona above such as: most of helium is not fully ionised below, but is above; and gas pressure and fluid dynamics seem to dominate below, however magnetic forces seem to dominate above.

The Corona is the next part of the Sun's Atmosphere. Unlike all the other layers of the Sun that get cooler the further they are from the core, the Corona actually gets much, much hotter than the visible surface of the Sun, although it is still not clear why. During quiet periods, the Corona is mainly gathered around the Sun's equator, however, during active periods, it spreads across different parts of the Sun's surface (particularly near sunspots).

And it is here that Solar Winds are formed, as superheated protons and electrons (‘smashed’ atom plasma) collect and leak out of the corona, at a rate of 7 billion tonnes per/hour, travelling at supersonic speeds.

Finally, the Heliosphere, is the outermost atmosphere of the Sun, which includes the solar wind plasma. It is described as an immense magnetic bubble-like region, which extends beyond Pluto, with its outer boundary, roughly defining the edge of the heliosphere and the interstellar gas outside the solar system.

Solar Activity: Coronal Mass Ejections, Coronal Loops, and Prominences

As well as a differing rotation speed between the Radiative Zone and the Convection Zone, the Sun also rotates at a different speed at the Equator than at the two Poles (due to it plasma like properties). This helps maintain the Magnetic Dynamo, and is also believed to twist the magnetic fields together, which over time, eventually erupt from within the Tachocline and through the Photosphere. These escaping magnetic fields form loops filled with plasma, known as Coronal Loops, and are thought to generate the Sun's intense Sunspots and Prominences.

The Sunspots appear dark on the Sun’s surface, as they are cooler than the surrounding plasma within the Photosphere (the high magnetic energy of the Coronal Loop inhibits convection), and also due to their large size (as large as 80,000km) they are visible from Earth. Prominences project out cool plasma into the Sun’s very hot exterior atmosphere – the Corona. Here they can break up and eject Gamma Rays, X-rays and Ultraviolet Rays out into space.

The Coronal Loops can also become twisted and finally break (known as Solar Flares) as they spontaneously reconfigure themselves into simpler forms, giving off energy that can also eject Gamma, X, and UV Rays into space. Of those very hot photons that were projected towards Earth, when they arrive, they can cause severe disruptions of the Earth’s upper atmosphere, creating such things as the Aurora Borealis (Northern Lights) and Geomagnetic Storms.

Coronal Mass Ejections (CME's) often follow from Solar Flares, and are usually large releases of plasma and magnetic fields from the Corona.Whilst Solar Flares are very fast, CME's are relatively slow - but have powerful effects on Earth's magnetosphere.