Distinguish between normal and anomalous dispersion
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The refractive index is a property of the medium through which light is passing. For a vacuum, the refractive index is 1 for all wavelengths.
Ordinary dielectric transparent media such as glass will have a higher index for blue than for red. (I don’t like to use the term “violet” because it is so easily confused with purple or magenta, which are not pure colors. Violet can be difficult to see and difficult to capture on a video camera or on certain kinds of film.)
The reason for the index change has to do with absorption bands in the glass. There are always some absorption bands in the infrared and in the ultra-violet. In between two absorption bands, the index always increases toward shorter wavelengths. In the absorption band itself, the index will decrease toward shorter wavelengths. This latter phenomenon is called “anomalous dispersion.” (or sometimes “resonant dispersion.”)
Newton noticed that every glass known in his time exhibited an increase in index as the color shifted from red to blue. He incorrectly concluded that dispersion could not be compensated by using multiple glasses and decided to build his telescopes using only mirrors.
All materials other than vacuum exhibit dispersion with wavelength because all materials absorb in some band or other. Absorption is a resonant phenomenon. Another resonant phenomenon is gain, such as in a laser, where a population inversion is present. In such a medium, the index may be higher for red than for blue. Other materials such as ionized gases, nanostructures, and specially made multi-layer dielectric coatings may exhibit anomalous refractive index properties under limited conditions. At longer wavelengths, such as radio waves, a number of materials have been built with special properties. In these types of media, the phase velocity can even exceed c, the velocity of light. In these cases, the refractive index may appear less than one. This does not violate relativity. Nothing real is actually traveling faster than c. No photons are actually traveling faster than c. This may be confusing but is a bit off topic so we won’t go into that.
When there is appreciable absorption present, the refractive index is actually complex. The real part is the ordinary refractive index, and the imaginary part is the absorption strength. There is a close relationship between the absorption versus wavelength and the index versus wavelength. Mathematically, it is known as the Kramers-Kronig relationship, which means any two media with identical absorption bands will have identical dispersions.
Crystalline materials are even more complicated because they can have different refractive indices in different directions. Such materials are called birefringent.
In summary, all ordinary clear transparent media such as plastics or glasses or salts exhibit dispersion so that the refractive index increases from red toward blue wavelengths. Only specially constructed “materials” will exhibit a reversal in the dispersion direction.
Ordinary dielectric transparent media such as glass will have a higher index for blue than for red. (I don’t like to use the term “violet” because it is so easily confused with purple or magenta, which are not pure colors. Violet can be difficult to see and difficult to capture on a video camera or on certain kinds of film.)
The reason for the index change has to do with absorption bands in the glass. There are always some absorption bands in the infrared and in the ultra-violet. In between two absorption bands, the index always increases toward shorter wavelengths. In the absorption band itself, the index will decrease toward shorter wavelengths. This latter phenomenon is called “anomalous dispersion.” (or sometimes “resonant dispersion.”)
Newton noticed that every glass known in his time exhibited an increase in index as the color shifted from red to blue. He incorrectly concluded that dispersion could not be compensated by using multiple glasses and decided to build his telescopes using only mirrors.
All materials other than vacuum exhibit dispersion with wavelength because all materials absorb in some band or other. Absorption is a resonant phenomenon. Another resonant phenomenon is gain, such as in a laser, where a population inversion is present. In such a medium, the index may be higher for red than for blue. Other materials such as ionized gases, nanostructures, and specially made multi-layer dielectric coatings may exhibit anomalous refractive index properties under limited conditions. At longer wavelengths, such as radio waves, a number of materials have been built with special properties. In these types of media, the phase velocity can even exceed c, the velocity of light. In these cases, the refractive index may appear less than one. This does not violate relativity. Nothing real is actually traveling faster than c. No photons are actually traveling faster than c. This may be confusing but is a bit off topic so we won’t go into that.
When there is appreciable absorption present, the refractive index is actually complex. The real part is the ordinary refractive index, and the imaginary part is the absorption strength. There is a close relationship between the absorption versus wavelength and the index versus wavelength. Mathematically, it is known as the Kramers-Kronig relationship, which means any two media with identical absorption bands will have identical dispersions.
Crystalline materials are even more complicated because they can have different refractive indices in different directions. Such materials are called birefringent.
In summary, all ordinary clear transparent media such as plastics or glasses or salts exhibit dispersion so that the refractive index increases from red toward blue wavelengths. Only specially constructed “materials” will exhibit a reversal in the dispersion direction.
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Normal dispersion:
Normal dispersion is dispersion where the group velocity declines with increasing optical frequency, occurs for many transparent media in the visible spectral region.
Anomalous dispersion:
Anomalous dispersion is a dispersion in which the normal order of the separation of components is reversed in the vicinity of particular wavelengths.
Hope it helped.......
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