What is visible light reflectivity

light

The title of this article is ambiguous. Further meanings are listed under light (disambiguation).
sunlight falling through the slats of a barn

light is a form of electromagnetic radiation. In the narrower sense, only those parts of the entire electromagnetic spectrum are meant that are visible to the human eye. In a broader sense, electromagnetic waves of shorter wavelength (ultraviolet) and longer wavelength (infrared) are also included.

The physical properties of light are described by different models: In ray optics, the straight propagation of light is illustrated by "light rays"; In wave optics, the wave nature of light is emphasized, which can also explain diffraction and interference phenomena. Finally, in quantum physics, light is described as a stream of quantum objects, the photons. Quantum electrodynamics offers a complete description of light. In a vacuum, light propagates at the constant speed of light of 299792458 m / s. When light hits matter, it can be scattered, reflected, refracted and slowed down or absorbed.

Light is the appropriate sensory stimulus for the human eye. The intensity of the light is perceived as brightness, the spectral composition as color.

The spectrum of electromagnetic radiation

history

Until well into modern times, it was largely unclear what light actually was. It was partly believed that the brightness fills the room without a time delay, and that "rays" emanate from the eyes and scan the environment during the visual process. However, there have been ideas since ancient times that light is emitted from the light source at a finite speed.

Galileo Galilei was one of the first to seriously attempt to measure the speed of light propagation, but to no avail. The funds available to him were far too crude for that. Only Ole Römer succeeded in doing this on the basis of observation data from Jupiter's moons 1676/78. The deviation of his measured value from the actual value (approx. 3 108 m / s) around 30%. The real achievement of Römers, however, was to prove that light propagates with finite speed. In the course of the following 200 years, Roman's measured value became more and more precise through more and more refined procedures (especially by Hippolyte Fizeau and Léon Foucault).

The nature of light, however, remained unexplained. In the 17th century, Isaac Newton tried to explain the propagation of light through the movement of small particles with his corpuscle theory. This made it possible to understand reflection, but not some other optical phenomena, such as diffraction, which is clearly a wave phenomenon. At the same time, Christiaan Huygens and others founded the wave theory of light, but it was not until the beginning of the 19th century that it became more and more popular after Thomas Young's double-slit experiments.

Michael Faraday was the first to prove in 1846 that light and magnetism are two interconnected physical phenomena. He published the magneto-optical effect he had found, now known as the Faraday effect[1] is referred to under the title About the magnetization of light and the exposure of the lines of magnetic force.[2]

In 1864 James Clerk Maxwell formulated the basic equations of electrodynamics that are still valid today and recognized that they predicted the existence of free electromagnetic waves. Since their predicted speed of propagation coincided with the known speed of light, he concluded that the light was probably an electromagnetic wave. He suspected (like almost all physicists at that time) that this wave could not exist in empty space, but needed a medium of propagation. This medium, which should fill the entire universe, was called ether.

With the electromagnetic light theory At the end of the 19th century, almost all questions about light seemed to have been resolved. However, on the one hand, the postulated ether could not be proven (see Michelson-Morley experiment), which ultimately opened the door to the special theory of relativity. On the other hand, among other things, the photo effect seemed to contradict the wave nature of light. This gave rise to a radically new way of looking at light, founded on the quantum hypothesis of Max Planck and Albert Einstein. The core of this hypothesis is the wave-particle dualism, which now describes light no longer exclusively as a wave or exclusively as a particle, but as a quantum object. As such, it unites the properties of waves and particles without being one or the other and thus eludes our concrete perception. This gave rise to quantum physics at the beginning of the 20th century and later quantum electrodynamics, which is still our understanding of the nature of light today.

science

physics

The most important models for describing light are presented below. Like all models in physics, the ones listed here are limited in their scope. As far as we know today, a complete description of the phenomenon “light” can only be provided by quantum electrodynamics.

Light as an electromagnetic wave

Linearly polarized electromagnetic wave in a vacuum. The monochromatic wave with wavelength $ \ lambda $ spreads x-Direction, the electric field strength $ \ vec E $ (in blue) and the magnetic flux density $ \ vec B $ (in red) are at right angles to each other and to the direction of propagation.

In classical electrodynamics, light is understood as a high-frequency electromagnetic wave. In a narrower sense, “light” is only that part of the electromagnetic spectrum that is visible to the human eye, i.e. wavelengths between approx. 380 and 780 nm. It is a transverse wave, the amplitude being given by the vector of the electric field or the magnetic field. The direction of propagation is perpendicular to it. The direction of the $ \ vec {E} $ field vector or $ \ vec {B} $ field vector is called the polarization direction. In the case of unpolarized light, the radiation field is composed of waves of all polarization directions. Like all electromagnetic waves, visible light also propagates in a vacuum with the speed of light of $ c \, = \, 299 \, 792 \, 458 \ \ frac {\ text {m}} {\ text {s}} $.

The wave equation of this electromagnetic wave can be derived from Maxwell's equations. This results in a simple relationship between the speed of light, the magnetic field constant $ \ mu_0 $ and the electric field constant $ \ varepsilon_0 $:

$ c = \ frac {1} {\ sqrt {\ mu_0 \ varepsilon_0}} $

in a vacuum,

$ c _ {\ text {Medium}} = \ frac {1} {\ sqrt {\ mu_0 \ mu_r \ varepsilon_0 \ varepsilon_r}} $

in the medium.

Obviously, the speed of light - more precisely: the phase speed of light - in media depends on their material properties. These can be summarized in the refractive index $ n $. In general, it is frequency dependent, which is known as dispersion. Among other things, this is the basis of the prism's ability to split light into its spectral components. With normal dispersion, short-wave blue light is refracted more strongly than long-wave red light.

Ray optics

The ray optics (also geometric optics) makes use of the approximation that the propagation of light can be illustrated by straight "rays". This approximation is justified above all when the dimensions of the test arrangement are large compared to the wavelength of the light. Then all diffraction phenomena can be neglected. The link between wave optics and ray optics is the wave vector, the direction of which corresponds to the direction of the light beam. Ray optics are particularly well suited to describing phenomena such as light and shadow, reflection or refraction. Therefore the function of many optical devices (pinhole camera, magnifying glass, telescope, microscope) can be explained with it. In particular, the imaging laws are also the basis for understanding the breaking apparatus in the human eye.

Principles of rays

  • Rays of light always propagate in a straight line and only change their direction when they hit a body (through reflection, refraction or scattering), regardless of the deflection of light by heavy masses (gravitational lens effect) observed in astronomy.
  • Rays of light can penetrate each other without influencing each other.
  • The light path is reversible. This means that every beam path would satisfy all optical laws even if the direction of propagation of the light were reversed.
reflection
Reflection and refraction at the interface between two transparent media of different optical density

On reflective surfaces (such as on bare metals), light is reflected according to the law of reflection. The incoming and outgoing beam as well as the perpendicular on the reflecting surface lie in one plane. The angle of incidence and the angle of reflection are equal to one another. The ratio of the reflected light intensity to the incident light intensity is referred to as the degree of reflection and is dependent on the material and the wavelength.

refraction

Light is refracted at the interface between two media of different optical density, i. that is, a ray changes direction at this interface. (For the sake of completeness, it should be said that at such a boundary surface the reflection always occurs more or less strongly.) Snellius' law of refraction states:

The incident and the refracted ray as well as the perpendicular on the interface lie in one plane. The angle between the perpendicular and the light beam is smaller in the medium that has the higher refractive index.

The exact angles $ \ delta_i $ can be calculated using the refractive indices $ n_i $ of the media involved:

$ n_1 \ sin (\ delta_1) = n_2 \ sin (\ delta_2) $.

If the incident ray from the optically denser medium hits the interface at a shallow angle, there is no real angle for the refracted ray that satisfies this condition. In this case, total reflection occurs instead of refraction.

Wave optics

Diffraction of a plane wave at a double slit: An elementary wave emanates from each of the two slits, both of which interfere with the typical diffraction pattern of a double slit.

Wave optics are based on the Huygens and Fresnel principle.

Every point of a wave front is the starting point of an elementary wave. A wave front results from the superposition of these elementary waves.

With Elemental wave In this context, a spherical wave is meant that emanates from a certain point. Wavefronts are the surfaces of the same phase. The distance between two successive wave fronts is thus the wavelength. The wave fronts of a plane wave are planes, the wave fronts of elementary waves are concentric spherical surfaces. The direction of propagation (i.e. the direction of the wave vector) always forms a normal to the wave front. With wave optics all phenomena of diffraction and interference can be understood. But it is also suitable for deriving the law of reflection and the law of refraction. The wave optics do not contradict the ray optics, but expand and deepen them.

Historically, the wave optics of Huygens and Fresnel already anticipated essential knowledge of electrodynamics: The light waves are electromagnetic waves.

Photons

In quantum physics, light is no longer understood as a classical wave, but as a quantum object. According to this, light is made up of individual discrete energy quanta, the so-called photons. A photon is an elementary particle, more precisely: a boson with a rest mass of 0, which always moves with the speed of light $ c $.

It carries an energy of

$ E = h \ nu $

Here $ \ nu $ is the frequency of light and $ h $ is Planck's quantum of action with $ h = 6 {,} 626 \, 069 \, 57 (29) \ cdot 10 ^ {- 34} \, \ text {Js} $.

The photon has a momentum of

$ p = \ frac {h} {\ lambda}, $

where $ \ lambda $ is the wavelength of light.

The spin of the photon is related to the polarization: the wave function of a single photon is circularly polarized. Depending on the direction of rotation of the $ \ vec E $ field vector, the spin of the photon is $ +1 $ or $ -1 $.

A photon is either absorbed and emitted as a whole or not at all. So it is “countable” like a particle. Nevertheless, everything that has been said here about the wave properties of light remains valid. The wave is described quantum mechanically correctly by a special case of the Klein-Gordon equation for massless particles (which corresponds to Maxwell's wave equation). This strange behavior of the photons, which is also shown by all other quantum objects, was described with the catchphrase “wave-particle dualism”: quantum objects are neither to be understood as classical particles nor as classical waves. Depending on the point of view, they show characteristics of one or the other.

In today's most common interpretation of quantum mechanics (Copenhagen interpretation) one cannot determine the exact location of a photon a priori predict. One can only make statements about the probability with which a photon will hit a certain point. This probability density is given by the square of the magnitude of the amplitude of the light wave.

Historically, the quantum mechanical description of light became necessary because some phenomena could not be explained with purely classical electrodynamics.

  • If one imagines a thermal light source (ideal case: black body) as a collection of many atomic oscillators that are in equilibrium with the radiation field, a classic derivation would lead to a "UV catastrophe", short-wave radiation would have to be in the spectrum of the black body be represented much more than it is. (Rayleigh Jeans Law)
  • Classical electrodynamics would predict that the energy of electrons released during the photoelectric effect is proportional to intensity of the absorbed radiation. In fact, it is (apart from a constant summand) proportional to frequency the radiation. This connection cannot be understood classically.
  • Sensitive detectors (e.g. photomultipliers) do not receive a constant, uniformly low intensity when the irradiation is weak, but rather individual signals that are very narrowly limited in terms of both space and time.
  • The spectrum of X-ray brake radiation has a short-wave limit that is directly related to the energy of the electrons that were used to generate them.

Interaction with matter

In addition to the phenomena already described earlier in this article

there are numerous other interactions between light and matter.

  • Absorption: The energy of the incident light is swallowed by a body. This can lead to an electron being raised to a higher energy level, the body to heat up. If the radiation is absorbed regardless of its wavelength, the body appears black. If only part of the spectrum is absorbed, the remaining parts of the spectrum determine the color of the body (subtractive color mixing). In the case of electronic excitation, the energy can also be emitted again in the form of radiation. One speaks of more spontaneous Emission, of fluorescence or - if the process is clearly delayed in time - of phosphorescence.
  • Birefringence: Some materials split a beam of light into two beams of different polarization.
  • Optical activity: Certain media can rotate the plane of polarization of polarized light.
  • Photo effect: The photons release electrons from the irradiated body.
  • Scattering: The light changes its propagation, however not in a defined direction as with the reflection, but diffusely in all possible spatial directions. Depending on the scattering body, a distinction is made between Compton scattering (on free electrons), Rayleigh scattering (on bound electrons without energy transfer), Raman scattering (on bound electrons with energy transfer), Mie scattering (on particles whose expansion is in the Order of magnitude of the wavelength).

Light sources

Continuous spectrum
Line spectrum (here: emission spectrum of hydrogen)

In principle, one differentiates between thermal and non-thermal Spotlights. The former obtain the energy for radiation emission from the thermal movement of their particles. Examples are candle flames, glowing bodies (filament of an incandescent lamp) or the sun. The spectrum of a thermal radiator is continuous; This means that all wavelengths occur, the spectral components depending exclusively on the temperature according to Planck's law of radiation, but not on the material of the radiator, apart from the spectral emissivity.

In contrast to this, non-thermal light sources do not have a continuous spectrum, but a line or band spectrum. This means that only very specific wavelengths are emitted.Line spectra occur with gas discharge tubes, band spectra with light-emitting diodes, polar lights or fireflies. The energy sources for the radiation are electric current, particle radiation or chemical reactions. Line spectra are often characteristic of certain substances.

The laser occupies a special position among the light sources. Laser light is almost monochromatic (it consists almost entirely of one wavelength), more or less coherent (there is a fixed phase relationship between several wave trains) and often polarized.

Cherenkov radiation results from the movement of charged particles through a transparent dielectric when the particle speed is higher than the speed of light in the dielectric. It is the analogue of the sonic boom and can be observed, for example, in swimming pool reactors and cooling pools in nuclear power plants.

Light receiver

  • The intact sense of sight is the simplest proof. Accordingly, the eye plays an important role in the direct observation of processes in which light is involved.
  • Photographic film plays a major role in researching the nature of light: Long exposure can be used to document the slightest light intensities of distant stars and their spectra. Photographic layers can be sensitized for different areas of the spectrum. In the meantime, however, photographic film is being replaced more and more by image sensors.
  • Optical radiation detectors mostly use the external (photocell, vidicon, image intensifier, photomultiplier) and internal (semiconductor detectors such as photodiode, phototransistor, photoresistor) photoelectric effect. Complex sensors (line sensors and image sensors), which also serve as recording elements in scanners and digital cameras, also work with semiconductor detectors. Color sensors work with several photodetectors behind different filters.
  • Fluorescence can be used to detect ultraviolet and infrared (after two-photon absorption) by evaluating the visible light that is generated.
  • Light can also be detected through its thermal effect. The bolometers used in astronomy to measure the radiation power of astronomical light sources and thermal power meters for high-power laser beams are based on this principle.

biology

Light as an eco-factor

Absorption spectrum of the green leaf pigment chlorophyll a and bwith which plants can absorb light and then use it; see also Soret band

In addition to the availability of water, light is the most important eco-factor for plants because it provides the energy for photosynthesis. The light energy absorbed by the chlorophyll molecules in the chloroplasts is used to split water molecules (photolysis) and thus produce reducing agents for photosynthesis. These are used in a second step to gradually reduce carbon dioxide to glucose, which is used to build starch, among other things. The oxygen produced during photolysis is released into the atmosphere as a residue. The sum reaction equation of photosynthesis is:

$ \ mathrm {6 \; CO_2 + 6 \; H_2O \ quad \ xrightarrow {h \ nu} \; C_6H_ {12} O_6 + 6 \; O_2} $

The structure of organic compounds from carbon dioxide is called carbon dioxide assimilation. Organisms that are able to do this with the help of light are called photo-autotrophic. In addition to vascular plants, mosses, algae and some bacteria, such as cyanobacteria and purple bacteria, are also included. All heterotrophic organisms are dependent on this assimilation because they can only meet their energy needs from organic compounds that they have to ingest with food.

The competition between plants for light becomes noticeable in the “storey structure” of the forest and the associated specialization of light and shadow plants or in the seasonal sequence of different aspects. Serves in waters just the light-flooded top layer, the nutrient layer, the formation of biomass and oxygen, mainly through phytoplankton. Because many animals and unicellular organisms find good living conditions here due to the high food supply and the comparatively high oxygen content of the water, they are attracted by the light.

Big Firefly (Lampyris noctiluca)

The sense of light or sight is one of the most important senses for many animals. It is used for orientation in space, for controlling the day-night rhythm, for recognizing dangers, for tracking down prey, for communicating with conspecifics. Therefore, in the course of evolution, the most diverse light-sensing organs have developed in the most varied of taxa. These range from simple eye spots Euglena, through simple pigment fields to complexly structured compound eyes and lens eyes. Only a few animals are completely insensitive to light stimuli. This is only the case when they live in complete darkness, like cave animals.

It is advantageous for both predator and prey animals Not to be seen. Adjustments to this are camouflage and night activity. Amazingly, on the other hand, many living beings have developed the ability to shine themselves. The most famous example is the firefly. This phenomenon of bioluminescence can also be found in deep-sea fish, luminescent crabs, fungi (Hallimasch) or bacteria. The benefits of bioluminescence are mainly explained with intra-species communication, deterring predators and attracting prey.

Light as a stimulus for the senses

Schematic longitudinal section through the human eye

The light that falls into the human eye is projected onto the retina by the breaking apparatus (consisting of the cornea, anterior and posterior chambers of the eye, lens and vitreous humor), where a real, upside-down image is created. (The process is comparable to that in a photo camera.) This stimulates the photoreceptors (= light sensory cells) in the retina, which convert the stimulus into an electrical signal. This signal is transmitted to the brain via the optic nerve, into which the individual nerve cords of the retina flow, where the sensation arises.

The light intensity is called brightness felt. The eye can adapt to the intensities - many powers of ten - through various mechanisms (see adaptation). The perceived brightness is related to the actual intensity via the Weber-Fechner law.

The spectral composition of the light stimulus is perceived as a sensation of color, whereby the human eye can detect light with wavelengths between approx. 380 nm and 750 nm. If white light is split up (by a prism), the wavelengths appear as the colors of the rainbow.

(approximately) shade Wavelength $ \ lambda $ in nm Wave frequency $ \ nu $ in THz Energy E per photon in eVWave number $ \ tilde {\ nu} $ in cm−1
violet 380–420 789,5–714,5 3,26–2,955 26.316–23.810
blue 420–490 714,5–612,5 2,95–2,535 23.810–20.408
green 490–575 612,5–522,5 2,53–2,165 20.408–17.391
yellow 575–585 522,5–513,5 2,16–2,125 17.391–17.094
orange 585–650 513,5–462,5 2,12–1,915 17.094–15.385
red 650–750 462,5–400,5 1,91–1,655 15.385–13.333

(It should be noted here, however, that, strictly speaking, this table only applies to monochromatic (i.e. single-colored) light. Mixed colors may produce completely different color impressions. For example, a mixed color of green and red monochromatic light appears yellow.)

Sensitivity distribution of human photoreceptors in rods (dashed in black) and the three types of cones (S, M and L).

The retina of the eye is equipped with various sensory cells: The rods have a broad spectral response and are characterized by a high level of sensitivity. They are therefore specialized in seeing at twilight, but cannot distinguish between colors. The cones, on the other hand, which are adapted to stronger intensities, come in three different types, each of which has its reaction optimum at a different wavelength. Their interconnection ultimately enables color vision.

The visual process in both rods and cones is based on the absorption of photons by the visual pigment (in the case of rods: rhodopsin). The retinal ligand undergoes isomerization, which causes the rhodopsin to break down and initiate the signal cascade of phototransduction. The resulting hyperpolarization of the cell membrane of the rods and cones produces an electrical signal that is passed on to the downstream nerve cells.

The services of the light sense organs of other living beings differ in part considerably from those of humans. While most mammals have a rather underdeveloped color vision, birds have more cone types and can accordingly distinguish more colors than humans. Bees are more or less insensitive to long-wave (red) light, but they can perceive very short-wave UV light, which is invisible to humans. They can also perceive the direction of polarization of the light. This helps them to orientate themselves in the room with the help of the sky blue. Some snakes, on the other hand, can perceive the IR rays, which are also invisible to us, with their pit organs.

chemistry

At organic dyes delocalized π-electrons can be raised to a higher level by frequencies in the visible range. As a result, certain wavelengths are absorbed depending on the molecule.

At inorganic dyes electrons from the d orbitals of an atom can also be excited into energetically higher d orbitals (see ligand field theory). Furthermore, electrons can change their position between central ion and ligand within a complex (see also charge transfer complexes and complex chemistry).

Sizes and units

Light measurement terms
  • The speed of light (c) is independent of the movement of the source and decreases in media compared to the speed of light in a vacuum. In a vacuum it is 299,792,458 meters per second and there is also independent of the movement of the observer.
  • The light color is determined by the wavelength-dependent composition of the light. This in turn is inversely proportional to the energy of the light quanta.
  • The polarization of the light describes the orientation of the electric and magnetic field vectors of the light in space. The light reflected flatly on dielectric surfaces and the light from the blue sky is partially linearly polarized, while the light from incandescent lamps and the sun does not have a preferred direction of polarization. Linear and circularly polarized light play a major role in optics and laser technology.
  • Luminous flux (lumens)
  • Amount of light (lumen second)
  • Light intensity (candela)
  • Luminance (candela / m²)
  • Illuminance (lux)
  • The light pressure (Newton second) is the physical force of light on particles or objects and, due to its small amount, only plays a noticeable role in weightlessness.
  • The color temperature (Kelvin) is the light color of a light source assigned to the temperature of a black body in order to classify it with regard to its color impression.
  • The light year (Lj, ly) is a unit of length used in astronomy that indicates the distance traveled by light during a year.

Light in society

Like fire, light is one of the most important phenomena for all cultures. Artificially generated light from lamps nowadays enables people to live a comfortable and safe life even in terrestrial darkness (night) and in covered rooms (caves, buildings). Technically, the functional group that generates light is called a lamp or illuminant. The holder for the lamp forms a light with this. "Light" and "lamp" are also used as symbols for intelligence (Bright spot, Education). A lack of intelligence is also referred to as "mental darkness" or "mental derangement". In Christianity, the light in the self-designation of Jesus Christ stands for the redemption of man from the darkness of distance from God. In the biblical creation story, light is the second work of God, after heaven and earth.

Year of Light 2015

In view of numerous anniversaries (e.g. Ibn Al HaythemsBook of seeing (1021), special and general theory of relativity (1905 and 1915) as well as the development of glass fiber by Charles Kao (1965)), the UNESCO has the year 2015 International Year of Light called out. All over the world events took place this year that deal with the importance of light for science and society. In Germany, the German Physical Society coordinated the activities for the Year of Light.[3]

Day of light

After the international year of light 2015, the UNESCO board of directors has the International day of light[4] (English International Day of Light[5]) called out. This will be formally announced at general conference in November 2017. The day of light is to be celebrated annually on May 16 from 2018.

Light from the perspective of the German legislator

As an environmental factor, light is one of the immissions in the sense of the Federal Immission Control Act (BImSchG). Light immissions from lighting systems can significantly disrupt the needs of people and animals to live and sleep and also hinder technical processes. Correspondingly, in the “light guidelines” of the federal states (in Germany) standards for assessing (room) brightening and (psychological) glare are specified. Intense colored or flashing light can be particularly disturbing. The environmental and pollution control authorities of the respective federal states are responsible for complaints. Negative effects relate to traffic safety (navigation at night, physiological glare from incorrectly adjusted headlights or from area lighting next to streets), influences on the animal world (attraction of nocturnal insects, disturbance of bird flight in migratory birds) and the general brightening of the earth's atmosphere (light pollution, astronomical observations due to the scattering of lamp light in the atmosphere of the night sky).

literature

  • Albert Einstein: About a heuristic point of view concerning the creation and transformation of light. In: Annals of Physics. 1905, pp. 132-148. With this contribution, Einstein established the wave-particle dualism of light.
  • Rolf Heilmann: Light. The fascinating story of a phenomenon, Herbig, Munich 2013, ISBN 978-3-7766-2711-4.
  • Klaus Hentschel: Einstein and the light quantum hypothesis. In: Scientific review. 58, 6, 2005, ISSN 0028-1050, pp. 311-319.
  • Thomas Walther, Herbert Walther: What is light From classic optics to quantum optics. Beck, Munich 1999, ISBN 3-406-44722-8.
  • Sidney Perkowitz: A brief history of light. Exploring a mystery. Deutscher Taschenbuch Verlag, Munich 1998, ISBN 3-423-33020-1.
  • George H. Rieke: Detection of Light - From the Ultraviolet to the Submillimeter. Cambridge Univ. Press, Cambridge 2003, ISBN 0-521-81636-X.
  • Wolfgang Schivelbusch: Lichtblicke: On the history of artificial brightness in the 19th century. Fischer Taschenbuch, Frankfurt am Main 2004, ISBN 978-3-596-16180-5.

Web links

 Wiktionary: light - Explanations of meanings, word origins, synonyms, translations
 Wikiquote: light - Quotes

Videos

Individual evidence

  1. ↑ Bergmann-Schaefer Textbook of Experimental Physics, 10th edition, page 906
  2. ↑ Michael Faraday: Experimental Researches in Electricity. Nineteenth Series. In: Philosophical Transactions of the Royal Society. Volume 136, 1846, pp. 1-20, doi: 10.1098 / rstl.1846.0001.
  3. ^ Official website for the Year of Light in Germany
  4. News from licht.de. (licht.de [accessed October 27, 2017]).
  5. Official website International Day of Light. Retrieved October 27, 2017.