Reflects glass light

Baunetz_Knowledge

Glass has an amorphous molecular structure without boundary layers and behaves optically isotropic as long as there are no internal stresses or external loads. However, glass is only permeable to part of the entire light spectrum. Part of the light is retained by absorption in the glass, another part is reflected back at the interface. The following applies to the degree of transmission t, the degree of absorption a and the degree of reflection r:

t + a + r = 1

In the case of commercially available flat glass, the degree of transmission is 83% - 90%, depending on the glass thickness; with normal incidence of light, about 8% of the light is reflected at the boundary surfaces. Commercially available glasses have a refractive index of approx. 1.5. The cause of the refraction is the change in speed of the light in the interface. The frequency of the light remains constant, only the wavelength changes.

The reflectivity can be specifically influenced by anti-reflective coatings with a defined refractive index or by fine etching of the surface. Due to the scattering of the light, the specular reflection then changes into a diffuse one. The absorption capacity of the glass is strongly dependent on its composition. Glasses in the visible spectral range are normally completely colorless. If the glass contains small amounts of components from subgroup elements (Cu, Ti, V, Cr, Mn, Fe, Co or Ni), electron jumps occur even with light with a relatively low energy, in which energy is absorbed. The glass thus shows discoloration in the visible area, which depends on the type of element. By using special admixtures, this effect can be used to color the glass in a targeted manner.

The commercially available flat glass is therefore almost impermeable to light in the ultraviolet range (UV). There is a pronounced absorption edge in the range of wavelengths λ = 150-250 nm. The position of this absorption edge shifts into the longer-wave range depending on the composition and alkali content. In the infrared range (IR), normal glass shows more or less pronounced absorption bands that are caused by an interaction of the light and components of the glass. All silicate glasses are impermeable to infrared rays with a wavelength above λ = 5,000 nm.

The absorption capacity in certain spectral ranges can be influenced in a targeted manner by means of special compositions of the glass or coatings. An example of this are sun protection glasses with the lowest possible transmission values ​​in the harmful infrared and ultraviolet spectral range.

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