# Monochromatic light

Monochromatic light (from the Greek mono-chromos , dt. "One color") is in the narrower sense monochrome visible light, in the general sense electromagnetic radiation of a precisely defined frequency or vacuum wavelength . In this context, one speaks of monochromatic waves . The color perception caused by monochromatic light in the visible spectral range is called its spectral color .

Perfect monochrome is an ideal that cannot be realized. A measure of the approximation to this ideal is the coherence length .

Almost monochromatic light can be produced in different ways: It can be filtered out of polychromatic light with its spectral distribution using a monochromator . On the other hand, devices can be used that emit light of atomic spectral lines .

## Monochromators

### prism

A prism splits light into spectral colors. If light with a suitable spectral distribution, for example daylight, passes through a prism, radiation components of different wavelengths are deflected to different degrees. Short-wave (blue) light is refracted more strongly than long-wave (red) light. Since the refractive index of the glass depends on the wavelength, the light emerges from the prism at a different angle depending on the wavelength.

### Diffraction grating

At diffraction gratings , these are fine grooves or threads of a certain thickness and a certain distance, incident light is diffracted in all directions. By interference an addition occurs in appropriate directions of diffraction of the beams to virtually monochromatic light.

Crystalline solids can be used to achieve monochromatic X-rays . The diffraction and interference takes place here at the individual, regularly arranged atoms of the crystal lattice.

### Optical filters

An optical filter is a selectively transparent material that is only transparent to light of certain wavelengths and absorbs other light. With the help of filters that are only transparent in a narrow range of wavelengths , almost monochromatic light can be generated. Such filters are often implemented as interference filters . The principle of Newton's rings is used here. A precisely defined arrangement of thin layers allows sufficient monochromatic light to be filtered out of polychromatic ( white ) light.

## Spectral line

Atoms or molecules can emit electromagnetic waves (photons) due to electron transitions between their energy levels . The energy difference between the levels involved determines the energy, and thus the wavelength, of the resulting radiation. According to Max Planck, the following applies here:

${\ displaystyle E = {\ frac {h \ cdot c} {\ lambda}}}$

The energy levels are quantized . Therefore there is only a finite number of transitions, correspondingly only a few wavelengths are represented. This creates a line spectrum during the emission .

A laser emits a narrowly defined wavelength that corresponds to the transition between two energy levels. Laser light of this spectral line is therefore approximately monochromatic.

Gas discharge lamps with low internal pressure also generate a line spectrum. The lines, which are dependent on the type of filling gas, have a high degree of constancy in their position and are spectrally very narrow. They are used to generate monochromatic radiation. Due to the color constancy, which is independent of external influences, such spectral lamps even serve as a wavelength standard.

## Individual evidence

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2. a b monochromator. In: Spektrum der Wissenschaft Verlagsgesellschaft mbH, Heidelberg, www.spektrum.de. 1998, accessed November 5, 2019 .
3. interference filter. In: Spektrum der Wissenschaft Verlagsgesellschaft mbH, Heidelberg, www.spektrum.de. 2019, accessed November 5, 2019 .
4. laser. In: Patrick Voss-de Haan, Spectrum of Science Verlagsgesellschaft mbH, Heidelberg, www.spektrum.de. 2019, accessed November 5, 2019 .
5. Spectral lamp . In: Spektrum der Wissenschaft Verlagsgesellschaft mbH, Heidelberg, www.spektrum.de. 2019, accessed November 5, 2019 .