Light is visible to the eye part of the electromagnetic radiation. In the electromagnetic spectrum, the range of light wavelengths from about 380 nm to 780 nm comprises representing frequencies of about 789 THz to 384 THz. An exact limit can not specify, since the sensitivity of the eye decreases to the borders of perception not abruptly but gradually. Adjacent to the visible light regions of the infrared ( wavelengths of between 780 nm and 1 mm) and the ultraviolet radiation ( wavelengths from 10 nm to 380 nm ) are frequently also referred to as light.

  • 2.2.1 Light as eco-factor
  • 2.2.2 Light as a sensory stimulus
  • 6.1 videos


Until well into the modern era was largely unclear what light actually is. It was believed in part that the brightness fills the space without time delay, and that " rays" emanating from the eyes and the environmental scan in vision. However, there have been around since ancient conceptions, according to which the light is emitted from the light source at a finite speed.

Galileo Galilei tried one of the first to measure the speed of propagation of light seriously, but without success. For him, the funds available were much too coarse. This became possible Ole Römer based on observation data of Jupiter's moons 1676/78. Although the deviation of its measured value from the actual value was (about 3 x 108 m / s) around 30%. The actual power Roman, however, was to prove that light travels at a finite speed. Roman's reading was over the next 200 years by increasingly sophisticated methods clarified (especially by Fizeau and Foucault) more and more.

The nature of light remained, however, still remains a mystery. Sir Isaac Newton tried in its corpuscular explaining the propagation of light by the movement of small particles. This could indeed understand the reflection, but not some other optical phenomena, such as diffraction. This is clearly a wave phenomenon. Christiaan Huygens and others founded the wave theory of light, which began to gain more and more especially after the double-slit experiment of Thomas Young.

James Clerk Maxwell formulated in 1864 which is still valid basic equations of electrodynamics and realized that by the existence of free electromagnetic waves was predicted. Since the predicted propagation velocity was consistent with the known speed of light, he concluded that the light was probably an electromagnetic wave. He assumed ( as it was then nearly all physicists ) that this wave could not exist in a vacuum, but would need a propagation medium. This medium would have to fill the entire universe was called the ether.

With the electromagnetic theory of light based on it seemed almost resolved all questions to light in the late 19th century. However, the postulated ether could not be detected on the one hand (see Michelson - Morley experiment ), which ultimately pushed open the door to special relativity. On the other hand, seemed among others, the photo effect of the wave nature of light to disagree. This created a radically new way of light, which was established by the quantum hypothesis of Max Planck and Albert Einstein. Core of this hypothesis is the wave -particle duality, which is now no longer describes the light only as a wave or as a particle only, but as a quantum object, which is neither the one nor the other and eludes our concrete perception. From the beginning of the 20th century was the quantum physics and later quantum electrodynamics, which represents our understanding of the nature of light to today.



The most important models are presented to describe the light. Like all models in physics are also listed here is limited in its scope. A complete according to our current knowledge description of the phenomenon " light " can only deliver the quantum electrodynamics.

Light as an electromagnetic wave

In classical electrodynamics, light is perceived as a high-frequency electromagnetic wave. In a narrower sense " light " only the visible to the human eye part of the electromagnetic spectrum, ie wavelengths between about 380 and 780 nm is It is a transverse wave, the amplitude is given by the vector of the electric field or the magnetic field. The direction of propagation is perpendicular to it. The direction of the field vector or field vector is called polarization. For unpolarized light, the radiation field from waves of all directions of polarization sets together. Like all electromagnetic waves and visible light propagates in vacuum with the speed of light from.

The wave equation of electromagnetic wave can be derived from the Maxwell equations. This results in a simple relationship between the speed of light and the magnetic field constant and the electric field constant.

In vacuo

In the medium.

Obviously, the speed of light depends - more precisely, the phase velocity of light - in the media of their material properties. These can be summarized in the refractive index. In general, it is frequency dependent, which is called dispersion. This is the basis, among others, the ability of a prism to split the light into its spectral components. Short-wave blue light is refracted more strongly than in normal dispersion long-wavelength red light.

Ray optics

Ray optics (also: geometrical optics) to make the approximation exploits the fact that the propagation of light can be illustrated by just "rays". This approximation is particularly justified if the dimensions of the experimental arrangement are large compared to the wavelength of light. Then, all the diffraction phenomena can be neglected. The link between wave optics and ray optics is the wave vector whose direction coincides with the direction of the light beam. Ray optics is particularly well suited to describe phenomena such as light and shadow, reflection or refraction. It can therefore be explained by their function of many optical devices ( pinhole camera, magnifying glass, telescope, microscope ). In particular, the imaging laws are also the basis for the understanding of the breaking apparatus in the human eye.

  • Light rays propagate always straight forward and change direction only when they encounter a body ( by reflection, refraction or scattering), disregarded the observed in astronomy deflection of light by heavy masses ( gravitational lensing ).
  • Light beams can pass through each other without affecting each other here.
  • The light path is reversed. This means that each beam path would also comply with all laws of optics, if you were to reverse the direction of light propagation.

On reflective surfaces (such as on bare metal) light is reflected according to the law of reflection.


Light is refracted at the interface between two media of different optical density, ie, a beam changes its direction at this interface. ( For completeness, let me say that always occurs at such an interface, the reflection more or less. ) States the refraction Snell's law:

The exact angles can be calculated by the refractive indices of the media involved:

If the incident beam from the optically denser medium impinges at a flat angle to the interface, there is no real angle for the diffracted beam which satisfies this condition. In this case, instead of refraction occurs in a total reflection.

Wave Optics

The wave optics is based on the principle of Huygens and Fresnel.

With wavelet is meant in this context a spherical wave emanating from a particular point. Wavefronts are surfaces of equal phase. The distance between two successive wave fronts is thus the wavelength. The wavefronts of a plane wave are so levels, the wave fronts of elementary waves are concentric spherical surfaces. The propagation direction ( ie the direction of the wave vector ) always forms a normal to the wave front. With the wave optics can be all the phenomena of diffraction and interference understand. But it is also suitable, the reflection and derive the law of refraction. The wave optics therefore does not contradict the ray optics, but broadens and deepens this.

Historically takes the wave optics of Huygens and Fresnel already significant findings of electrodynamics in advance: The light waves are electromagnetic waves.


In quantum physics, light is seen no longer as a classical wave, but as a quantum object. Accordingly, the light from individual discrete energy quanta, called photons sets together. A photon is an elementary particle, or more precisely a boson with a rest mass of 0, which always moves at the speed of light.

It transmits an energy of

Here, with the frequency of light and Planck's constant.

The photon has a momentum of

Wherein the wavelength of the light.

The spinning of the photon is related to the polarization: The wave function of a single photon is circularly polarized. Depending on the rotation direction of the field vector is the spin of the photon or.

A photon is either absorbed and emitted as a whole or not at all. So it is " countable " as a particle. Nevertheless, there remains everything that has been said here about the wave properties of light, valid. The shaft is described quantum mechanically correct by a special case of the Klein-Gordon equation for massless particles (which Maxwell's wave equation corresponds ). This strange behavior of the photons, but the show also all other quantum objects, was designated the keyword " wave -particle duality ": quantum objects are still to be understood neither as classical particles like classical waves. Depending on the approach, they show characteristics of one or the other.

In today's popular interpretation of quantum mechanics ( the Copenhagen interpretation ) can be the exact location of a photon is not a priori predict. One can only make statements about the probability of a photon is incident at a particular location. This probability density is given by the squared modulus of the amplitude of the light wave.

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

  • If you imagine a thermal light source ( ideally: Black body ) as an accumulation of many atomic oscillators before, related to the radiation field in equilibrium, a classical derivation for would lead " UV catastrophe," short-wave radiation would in the spectrum of the black body be much better represented than it is. ( Rayleigh-Jeans law )
  • Classical Electrodynamics would predict that the energy of electrons, which are released when the photoelectric effect is proportional to the intensity of the absorbed radiation. But in fact it is ( except for a constant term ) proportional to the frequency of the radiation. This relationship can not be understood classically.
  • Sensitive detectors (eg photomultipliers ) received over low radiation rather than a constant uniformly low intensity, but individual, both spatially and temporally very restricted signals.
  • The spectrum of X-ray radiation has a short wavelength limit, which is directly related to the energy of the electrons together, which were used for their production.

Interaction with matter





Optical activity

Photoelectric effect

In addition to the phenomena already described earlier in this article

  • Refraction
  • Reflection

There are many other interactions between light and matter.

  • Absorption: the energy of the incident light is swallowed by a body. This can cause an electron is raised to a higher energy level that warms the body. If the radiation is absorbed regardless of their wavelength, the body appears black. Only a portion of the spectrum is absorbed, determine the remaining parts of the spectrum, the color of the body ( subtractive color mixing). In case of electronic excitation energy can be released back into the form of radiation. This is called spontaneous emission, fluorescence, or - if the operation is performed significantly delayed - of phosphorescence.
  • Birefringence: Some materials split a light beam into two beams of different polarization.
  • Optical Activity: Certain media can rotate the plane of polarization of polarized light.
  • Photo Effect: The photons resolve electrons from the irradiated body.
  • Scattering: The light does not change its diffusion, but as in the reflection in a defined direction, but diffusely in all possible spatial directions. Depending on the scattering body distinction is made between Compton scattering ( of free electrons), Rayleigh scattering ( of bound electrons without energy transfer ), Raman scattering ( of bound electrons with energy transfer ), Mie scattering ( of particles whose dimension in the order of the wavelength is ).

Light sources

A basic distinction between thermal and non - thermal emitters. The former relate the energy for the emission of radiation from the thermal motion of its particles. Examples would be a burning candle, the filament of a conventional bulb or the sun. The spectrum of a thermal radiator is continuous, ie it occurs all wavelengths, with the spectral components according to Planck's radiation law, depend exclusively on the temperature, but does not depend on the material of the radiator.

In contrast, non - thermal light sources do not have a continuous spectrum, but a line or a band spectrum. This means that only certain wavelengths are emitted. Line spectra occur in gas discharge tubes, band spectra of LEDs, polar lights or fireflies. The power source for the radiation are here usually electric currents or discharge or chemical reactions. Line spectra are often characteristic of certain substances.

A special position among the light sources is occupied by the laser. Laser light is monochromatic ( it consists of only one wavelength), coherent (there is a fixed phase relationship between all wave trains ), usually polarized and extremely well focused.

Light receiver

  • The intact sense of sight is the easiest proof. Accordingly, the eye plays an important role in the direct observation of events in which light is involved.
  • The photographic film played in the exploration of the nature of light a big role: you could by long exposure lowest light intensities of distant stars and their spectra recorded. Photographic layers can be sensitized for different regions of the spectrum.
  • Optical radiation detectors usually use the outer ( photocell, vidicon, image intensifiers, photomultipliers ) and internal ( semiconductor detectors such as photodiode, phototransistor, photoresistor ) photoelectric effect. Complex sensors ( line sensors and surface sensors), which are also used in scanners and digital cameras as the recording element, are also working with semiconductor detectors. Color sensors work with several, behind different filters photodetectors.
  • By fluorescence and photoluminescence can be infrared and ultraviolet light can be detected by the visible light generated is evaluated.
  • Light can be detected by its thermal effect. Based on this principle, the bolometer used in astronomy to measure the radiation power of astronomical light sources, as well as thermal power meters for high power laser beams.


Light as eco-factor

Light represents for plants - in addition to the availability of water - the most important eco-factor is because it provides the energy for photosynthesis. Absorbed by the chlorophyll molecules in the chloroplasts, light energy is used to produce split water molecules ( photolysis ), and as reducing agents for the photosynthesis. These are used in a second step, to progressively reduce carbon dioxide to eventually glucose, resulting in, inter alia, strength is built up. The obtained in the photolysis of oxygen is discharged as residue to the atmosphere. The overall reaction equation of photosynthesis is:

The structure of organic compounds from carbon dioxide is called the carbon dioxide assimilation. Organisms that are to using light in the situation, it is called photo - autotrophic. In addition to the vascular plants including the mosses, algae and some bacteria, such as cyanobacteria and purple bacteria, do so. All heterotrophic organisms depend on this assimilation because they can meet their energy requirements only from organic compounds, which must include them in the diet.

The competition of plants for light manifests itself in the " floor construction " of the forest and the associated specialization of light and shade plants or in the year time series of different aspects. In waters only the light-filled top layer, the feeder layer, the formation of biomass and oxygen is used, mainly by phytoplankton. Because many animals and single-celled organisms found by the high food availability and the relatively high oxygen content of the water here, good living conditions, they are attracted by the light.

The light or sense of sight is one of the most important sense for many animals. It is used for orientation in space, to control the day -night rhythm, to recognize danger, to detect prey, communicate with conspecifics. Therefore, the variety of light sensory organs have evolved in the course of evolution in a variety of taxa. These range from simple eyespots of Euglena, simple pigment fields to the complex structure compound eyes and lens eyes. Few animals are completely insensitive to light stimuli. This is more than the case if they live in total darkness, like cave animals.

For both predator- prey and it is an advantage not to be seen. Adjustments to it are camouflage and night activity. Amazingly, then, to have many creatures to glow even developed the ability. The most famous example is the firefly. We find this phenomenon of bioluminescence but also in deep sea fishing, light crabs, fungi ( Armillaria ) or bacteria. The benefits of bioluminescence is explained primarily with intraspecific communication, deterring predators, attracting prey.

Light as a sensory stimulus

The light that falls into the human eye, by the crushing apparatus (consisting of cornea, anterior and posterior chamber of the eye, lens and vitreous body ) projected on the retina, where a real, upside-down image. ( The process is similar to that in a camera comparable. ) Thus, the present in the retinal photoreceptors ( = light-sensitive cells ) are stimulated, which translate the stimulus into an electrical signal. This signal is fed via the optic nerve to the individual nerve fibers of the retina to the brain, where perception occurs.

The light intensity is perceived as brightness. The eye can be adapted by various mechanisms of intensities, some of which many orders of magnitude apart (see adaptation). The perceived brightness depends with the actual intensity together over the Weber- Fechner law.

The spectral composition of the light stimulus is perceived as a color perception, the human eye is sensitive to light having wavelengths between about 380 nm and 750 nm. If one separates white light ( through a prism ) according to the wavelengths so appear all the colors of the rainbow.

( However, it should be noted that, strictly speaking, only for monochromatic (ie, single color ) light this table applies. Mixed colors call may completely different color impressions forth. Such a mixed color of green and red monochromatic light, for example yellow appears. )

The retina of the eye is equipped with various sensory cells: The rods have a broad spectral responsiveness to and are characterized by a high sensitivity. Therefore, they are specialized for seeing at dusk, but can not distinguish colors. The pin, however, are adapted to more intensities are found in three types, which have their respective optimum reaction at a different wavelength. Your wiring will eventually allow for color vision.

Both the rod and the pin in the process of vision based on the absorption of photons by the visual pigment (in the case of the rod: rhodopsin ). The ligand retinal makes up by a conformational change that results in the rhodopsin breaks down and the signal cascade of Fototransduktion in motion. Caused thereby hyperpolarization of the cell membrane of the rods and cones causes electrical signal which is passed on to the downstream neurons. (see visual pathway )

The powers of the light sense organs of other organisms sometimes differ significantly from those of humans. While most mammals have a rather under -developed color vision, birds have more cone types and accordingly may differ more colors than humans. Although bees are more or less insensitive to long-wavelength (red ) light, but can perceive the very short-wave UV light, which is invisible to humans. They can also perceive the polarization direction of light. This helps them in the orientation in space using the sky blue. Some snakes can also turn invisible for us IR rays perceive with their pit organs.


For organic dyes delocalized π - electrons can be lifted by frequencies in the visible range to a higher level. This certain wavelengths are absorbed depending on the molecule.

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

Quantities and units

  • The speed of light ( c) is independent of the movement of the source and decreases in the media with respect to the vacuum light speed. It is under vacuum 299,792,458 meters per second and there is also independent of the movement of the observer.
  • The light color is determined by the wellenlängeabgängigen composition of light. This in turn is inversely proportional to the energy of photons.
  • The polarization of the light describes the orientation of the electric and magnetic field vectors of the light in the room. The flat surfaces of the light reflected at the dielectric, and the light of the blue sky is partially linearly polarized as the light of incandescent lamps and the sun does not have a preferred direction of polarization. Linear and circular polarized light play an important role in optics and laser technology.
  • Luminous flux ( lumens)
  • Amount of light ( lumens second)
  • Luminous intensity ( candela)
  • Luminance (candela / m²)
  • Illuminance ( lux)
  • The light pressure ( Newton seconds ) is the physical force effect of light on particles or objects and by virtue of its small amount only in zero gravity a significant role.
  • The color temperature (Kelvin ) is the temperature of a black body radiator associated light color of a light source in order to classify these in terms of their color impression.
  • The light-year ( ly, ly ) is a unit of length used in astronomy, indicating the course of a year traveled by the light path.

Light in society

Is light, like fire, one of the most important phenomena for all cultures. Artificially generated light from lamps allows the people nowadays a comfortable and secure life even in terrestrial darkness ( night) and in closed spaces (caves, buildings). Technically, the functional group which produces light of a lamp or light-emitting means respectively. The holder for the lamp is a lamp with this. " Light" and " light" are also used as symbols of intelligence ( bright spot, reconnaissance). A lack of intelligence is also referred to as " spiritual darkness " or " insanity ". In Christianity, the light is in the self- designation of Jesus Christ for the redemption of men from the darkness of separation from God. In the biblical story of creation, the light is the first work of God.

Light from the perspective of the German legislature

Light counts as an environmental factor to the emissions pursuant to the Federal Pollution Control Act ( Federal Pollution Control Act ). Light emissions of lighting systems can significantly disturb the living and sleeping needs of humans and animals and also hinder technical processes. Accordingly, the countries are in the " light " Directive standards for the evaluation of ( space - ) lightening and the ( psychological ) determined glare ( in Germany ). Particularly disturbing to act intensively colored or blinking light. Responsible are the environmental and pollution control authorities of the federal states in case of complaints. Negative effects relate to road safety (navigation at night, physiological glare due to incorrectly set car headlights or area lights next to roads), impacts on wildlife ( tightening of nocturnal insects, disturbance of flying birds in migratory birds ) and the general lightening of the atmosphere ( light pollution, the astronomical observations due to scattering of the lamp light in the atmosphere of the night sky Disabled ).