Definition "Light"
Visible light is the spectrum of electromagnetic radiation that can be detected by the human eye, i.e. electromagnetic waves with wavelengths ranging from about 380 to 780 nanometres (nm).
Physiology
The subject of photometry deals with the different sensitivity of pigment molecules (blue, green-yellow, orange-red) in different types of cone and rod photoreceptor cells in the human eye for different wavelengths (V(λ)- curve). Whereas the cones are responsible for colour vision, the rods in the retina register the light intensity with the retinal molecules by way of rhodopsin separation.
The colour effect of the physiological act of seeing is produced by the absorption of individual wavelengths by a particular colour. When particular wavelengths are absorbed, a colour impression (complementary colour) is produced from the remaining wavelengths. Thus a green leaf does not absorb wavelengths from the "green" spectrum but from the complementary "red" (680nm) and "blue" (430nm) spectra.
The light occurring in our surroundings is a mixture of different wavelengths. This polychromatic (multicolour) light can be separated into its monochromatic (single colour) components using a diffraction grating or a prism. Each one of these monochromatic components of light corresponds to a specific colour impression for humans, i.e. the so-called "spectral colours" or the "colours of the rainbow".
The following spectral colours can be discerned, listed in order of increasing wavelength.
| Colour | Wavelength | Wave frequency | Energy per photon |
|---|---|---|---|
| Violet | 380 - 420nm | 789 - 714THz | 3.26 - 2.95eV |
| Blue | 420 - 490nm | 714 - 612THz | 2.95 - 2.53eV |
| Green | 490 - 575nm | 612 - 522THz | 2.53 - 2.16eV |
| Yellow | 575 - 585nm | 522 - 513THz | 2.16 - 2.12eV |
| Orange | 585 - 650nm | 513 - 462THz | 2.12 - 1.91eV |
| Red | 650 - 750nm | 462 - 400THz | 1.91 - 1.65eV |
- Click to enlarge
The changes from one colour to the next are flowing transitions. A person's own particular impression of an individual denotable, quantifiable colour is subjective and conditioned by language, tradition and thought. This can be seen from the different words (originally) given to colours in different languages.
The individual colour spectra each contain different colour hues. Thus for instance the spectrum between blue and green is called turquoise or cyan. Other perceived colours (such as brown) are produced from light in which several wavelengths occur (additive colour mixing) or by subtractive colour mixing from filtered white light.
Electromagnetic radiation beyond the spectrum visible to humans with a wavelength lower than violet is referred to as ultra-violet or UV-radiation up to a certain frequency, whereas that with a wavelength larger than red is referred to as infrared or IR-radiation up to a certain wavelength. The bandwidth of light visible to animals can vary considerably from that visible to humans.
Physics
One of the main sources of light is the sun. Artificial light sources include incandescent lamps, fluorescent tubes, LEDs, lasers and chemical light.
In the history of physics light has partly been viewed as a wave phenomenon, based on empirical findings, and partly, based on other experimental findings, as a flow of particles. This wave-particle dualism is explained by quantum physics in the sense that light is both. The same applies to all particle radiation.
The light particles (known as "photons") have no rest mass and in a vacuum always travel at the speed of light "c". The speed of light has a crucial role to play in the theory of relativity. It also gives us the proportionality factor between energy "E" and mass "m" in Einstein's famous formula: E = m · c².
Furthermore, the relationship between the frequency "f" and the energy "E" of the photon in question is given by Planck's constant "h" in the formula E = h · f.
According to the theory of relativity, light is subject in general to gravitation and is bent or deflected in a gravitational field. For the same reason, it also carries its own weak gravitational field itself, as can be demonstrated in a ring laser for instance. By using the effects of relativity, light can therefore be slowed down and influenced by gravitation.
When electrons in an atom jump from a higher to a lower energy level, light is emitted, the frequency of which lies in the visible (also known as luminescence) or in the infrared range (see also infrared radiation).
In addition to the colour, i.e. the wavelength and frequency, light is also characterised by coherence, interference and polarisation and by further measurable parameters.
Objects that are completely translucent, i.e. that let all the light through, are known as transparent. Limited translucence (non-transparent) objects are known as "opaque" (see "opacity"). Impermeability to light is known as non-transparent.
Light in the darkness
Light under the open sky can assist terrestrial navigation (pedestrians, car drivers) by acting as an optical signal or it can be used for decorative and advertising purposes. As an environmental factor, it is one of the emissions that comes under the German Federal Emission Control Act (BImSchG). Light emissions from lighting systems can considerably disturb the waking and sleeping patterns of people and animals and can also prevent technical processes.
Accordingly, various standards have been determined for judging the (spatial) brightening and the (psychological) dazzle in the so-called "Light Directive" of the states (in Germany). Intensive coloured or flashing light can be particularly disturbing. The environmental and emission control authorities of the states (in Germany) are responsible for dealing with any complaints received.
Negative effects concern the traffic safety (navigation at night, physiological dazzle, e.g. by incorrectly adjusted car headlights or by illumination of areas beside the roads).
The influences on the animal kingdom include aspects such as: attracting nocturnal insects, disturbing the flight of migratory birds and generally brightening up the atmosphere (i.e. light pollution, whereby astronomic observation is rendered impossible due to the dispersion of lamp light in the atmosphere of the night sky).