Can convert phosphorus visible light into UV


Lexicon> letter L> light

Definition: electromagnetic radiation in a certain range of wavelengths or frequencies

English: light

Categories: Basic Concepts, Physical Basics

Author: Dr. Rüdiger Paschotta

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Original creation: 07/20/2014; last change: 03/14/2020


Light is a type of electromagnetic radiation; H. a phenomenon of electromagnetism. Only radiation that is visible to the human eye is considered to be light in the narrower sense. This is the case at wavelengths between approx. 400 nm and 750 nm, or at frequencies between approx. 400 THz and 750 THz. (1 THz means 1012 = one trillion oscillations per second.) Since the visibility of the light also depends on the respective eye and on the intensity, the mentioned wavelength range is slightly different from case to case. The closer the wavelength approaches the limits mentioned, the more intensity is required so that something can still be seen.

Longer-wave radiation (with wavelengths above 750 nm) is called in a broader sense Infrared light (IR light) refers to the extent that it still behaves physically similar to visible light (e.g. with regard to its propagation in air). There is no generally accepted upper limit for the wavelength of infrared light. B. start at 1 mm. Electromagnetic radiation with even longer waves falls into the range of terahertz radiation, millimeter waves, microwaves, radio waves, etc.

On the other hand one speaks of ultraviolet light (UV light) if the wavelength is below approx. 400 nm. Here, too, there is no sharply defined limit for the wavelength; one finally gets into the realms of x-rays and gamma rays (a type of radioactive radiation).

Light as a form of energy

Like other types of electromagnetic radiation, light transports energy. For example, the sun sends a lot of energy to the earth in this form, although the lion's share of it comes from infrared light. Since infrared light contributes most of the warming from sunlight, it is also known as thermal radiation.

The energy of light is quantized, i.e. H. it will give off in the form of very small packets of energy. Their size depends on the respective frequency or wavelength. According to a naive idea, light is simply a stream of small packets of energy, a type of particle known as Photons are designated. This idea is consistent with some observations but not others. In particular, light also has a wave character which manifests itself in interference phenomena and which would not be expected with a simple beam of particles. This so-called wave-particle dualism can hardly be grasped with simple pictorial ideas, but is described in a logically consistent manner within the framework of quantum theory. Quantum theory reproduces the observable behavior of light extremely precisely and reliably.

Light is a form of energy that can hardly be stored. Although you can in principle in a so-called light optical resonator where it is guided in a closed path using highly reflective mirrors. However, even when using extremely highly reflective mirrors, the energy is lost very quickly (usually within much less than a second), since the light propagates at extremely high speed (for example in air at approx. 300,000 km / s) and because of its At least a small part of the energy is lost with each reflection.

Generation of light

Light from different sources can differ in many ways.

The sunlight mentioned above contains a very wide range of wavelengths and frequencies. The same applies to other sources that emit light due to their very high temperature (thermal emitters). If the temperature of an object is not so high that significant amounts of visible light can be emitted, some thermal radiation (infrared light) can still be generated.

Light can also arise in electrical gas discharges, where atoms or molecules can get into so-called excited states, from which they often return to their basic state when light is emitted. This light transports energy that was previously briefly contained in the atoms or molecules as so-called excitation energy. In contrast to thermal radiation, this light often, but not always, preferably contains wavelengths in very specific narrow ranges. The so-called light spectrum is not continuous, but consists of discrete lines that correspond to certain wavelengths.

Light, which mainly contains a narrow range of wavelengths, is perceived by the eye as colored. For example, light with wavelengths around 500 nm looks green, while red light has wavelengths above 600 nm. Light with a balanced mixture of wavelengths (i.e. with a continuous spectrum or with several spectral lines) can appear white to the eye.

Incandescent lamps generate thermal radiation that has a continuous spectrum and only a few percent of the energy falls within the range of visible light. On the other hand, gas discharge lamps can generate directly visible light with higher efficiency, or ultraviolet light, which can then be converted into visible light in a fluorescent material (“phosphor”) (→Fluorescent lamp). Another type of light generation takes place in light-emitting diodes, where charge carriers in a semiconductor structure are electrically excited and can thus emit light. Here, too, the radiation takes place in a narrow wavelength range and mostly with high efficiency.

Other technically important sources of light (mostly infrared light) are laser. These often generate light that is concentrated on a single narrow wavelength range. Another important difference to sunlight, for example, is that laser light has very regularly shaped wave fronts. This enables the very directional propagation of light beams with only little preparation, even over greater distances, and also enables the light to be focused on a very small area. In addition, lasers can deliver the energy in the form of very short light pulses, i.e. H. that the light energy is also very strongly concentrated in time. Laser light therefore offers the possibility of administering energy in an extremely targeted manner in terms of space and time. This is used, for example, in various material processing methods.

Conversion of light into other forms of energy

For example, when light is absorbed on a solid body, its energy is usually converted into heat. This is used, for example, in solar collectors (→Solar thermal). In some cases, however, other forms of energy can also arise. For example, a solar cell can generate electrical energy directly from light (→Photovoltaic). The technical use of the energy of sunlight for the generation of electrical energy or heat is one of the most important types of use of renewable energy.

Light can also trigger chemical transformations, and not just by generating heat. This is particularly the case with short-wave light, i.e. with ultraviolet light much sooner than with visible or even infrared light. This is related to the fact that the excitation (or also ionization) of atoms or molecules requires a certain amount of energy, i.e. the energy of the photons must be sufficiently high. This is only the case with a sufficiently high frequency or a sufficiently small wavelength.

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See also: lighting, thermal radiation, radiation
as well as other articles in the categories basic concepts, physical fundamentals