The reststrahlen effect (German: “residual rays”) is a
reflectance
The reflectance of the surface of a material is its effectiveness in Reflection (physics), reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the respon ...
phenomenon in which
electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible ...
within a narrow energy band cannot propagate within a given medium due to a change in
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, o ...
concurrent with the specific absorbance band of the medium in question; this narrow energy band is termed the ''reststrahlen band''.
As a result of this inability to propagate, normally incident reststrahlen band radiation experiences strong-reflection or total-reflection from that medium.
The energies at which reststrahlen bands occur vary and are particular to the individual compound.
Numerous physical attributes of a compound will have an effect on the appearance of the reststrahlen band. These include phonon band-gap, particle/grain size, strongly absorbing compounds, compounds with optically opaque bands in the infrared.
Appearance
"The term Reststrahlen was coined following the observation by
Heinrich Rubens
Heinrich Rubens (30 March 1865, Wiesbaden, Nassau, Germany – 17 July 1922, Berlin, Germany) was a German physicist. He is known for his measurements of the energy of black-body radiation which led Max Planck to the discovery of his radiati ...
(more than a century ago) that repeated reflection of an infrared beam at the surface of a given material suppresses
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, vi ...
at all wavelengths except for certain spectral intervals. The measured intensity for these special intervals (the Reststrahlen range) indicates a
reflectance
The reflectance of the surface of a material is its effectiveness in Reflection (physics), reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the respon ...
of up to 80% or even more, while the maximum
reflectance
The reflectance of the surface of a material is its effectiveness in Reflection (physics), reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the respon ...
due to infrared bands of dielectric materials are usually <10%. After four reflections, the intensity of the latter is reduced by a factor of 10
−4 compared to the intensity of the incident radiation, while the light in the Reststrahlen range can maintain 40% of its original intensity by the time it reaches the detector. Obviously, this contrast increases with the number of reflections and explains the observation made by Rubens and the term Reststrahlen (residual rays) used to describe this spectral selection."
Reststrahlen bands manifest in diffuse reflectance
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...
absorption spectra as complete band reversal, or in infrared emission spectra as a minimum in emissivity.
Application
The reststrahlen effect is used to investigate the properties of semiconductors, it is also used in
geophysics
Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' som ...
and
meteorology
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...
.
See also
*
Absorbance
Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative lo ...
*
Emissivity
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...
*
Transmittance
Transmittance of the surface of a material is its effectiveness in transmitting radiant energy. It is the fraction of incident electromagnetic power that is transmitted through a sample, in contrast to the transmission coefficient, which is th ...
*
Reflectivity
The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electroni ...
*
Lyddane–Sachs–Teller relation
In condensed matter physics, the Lyddane–Sachs–Teller relation (or LST relation) determines the ratio of the natural frequency of longitudinal optic lattice vibrations (phonons) (\omega_\text) of an ionic crystal to the natural frequency of the ...
References
{{Reflist
* Elachi, C. ''et al.'' (2006) ''Introduction to the physics and techniques of remote sensing''. John Wiley and Sons.
* Griffiths, P.R. (1983) ''Fourier transform infrared spectrometry''. Science, 222, 297–302.
* Goldberg, A. ''et al.'' (2003) ''Detection of buried land mines using a dual-band LWIR/LWIR QWIP focal plane array''. Infrared Physics & Technology, 44 (5–6), 427–437.
* Anderson, M. S. ''et al.'' (2005) ''Fourier transform infrared spectroscopy for Mars science''. Rev. Sci. Instrum., 76 (3).
Spectroscopy
Infrared technology
Scientific techniques