4 ms·

Fluorite just has low dispersion and therefore a lens made from it has less chromatic aberration than glass.

davidmurdoch 3y ago
> the red to green wavelengths are dispersed with the same tendencies as glass, but the green to blue wavelengths are dispersed more than glass.
- rainbowzootsuit 3y ago
  I admittedly responded without reading the source, so please accept my apologies. Despite how it is worded the total dispersion of Fluorite is less than glass. That quote is referring to the ‘extraordinary partial dispersion’ property. They are using this to better correct the aberration than could be done with two pieces of glass alone. This seems to be illustrated in the diagrams well. Some further stuff to read up on is the Abbe number that describes the refractive index vs wavelength derived from a set of light sources. https://en.wikipedia.org/wiki/Abbe_number https://en.wikipedia.org/wiki/Abbe_number The higher the Abbe number the lower the dispersion of a material. Fluorite is ~95 https://refractiveindex.info/?shelf=main&book=CaF2&page=Malitson https://refractiveindex.info/?shelf=main&book=CaF2&page=Mali... Random choice below but glass tends to be ~25 - 80 https://refractiveindex.info/?shelf=glass&book=HOYA-C&page=E-C3 https://refractiveindex.info/?shelf=glass&book=HOYA-C&page=E...
foobar1962 3y ago
> Fluorite just has low dispersion and therefore a lens made from it has less chromatic aberration than glass. So why not make the entire lens from fluorite? Because pairing an element with one with different refractive index can lower the overall dispersion. Typically you'll see compound lenses are made of pairs of elements where one is positive (convex) and the other negative (concave): instead of making one lens with power of, say, +4, the group is made from one that's +5 and one that's -1 of different type of glass so the refractive indexes cancel-out dispersion and other aberrations.

> the red to green wavelengths are dispersed with the same tendencies as glass, but the green to blue wavelengths are dispersed more than glass.

rainbowzootsuit 3y ago
I admittedly responded without reading the source, so please accept my apologies. Despite how it is worded the total dispersion of Fluorite is less than glass. That quote is referring to the ‘extraordinary partial dispersion’ property. They are using this to better correct the aberration than could be done with two pieces of glass alone. This seems to be illustrated in the diagrams well. Some further stuff to read up on is the Abbe number that describes the refractive index vs wavelength derived from a set of light sources. https://en.wikipedia.org/wiki/Abbe_number https://en.wikipedia.org/wiki/Abbe_number The higher the Abbe number the lower the dispersion of a material. Fluorite is ~95 https://refractiveindex.info/?shelf=main&book=CaF2&page=Malitson https://refractiveindex.info/?shelf=main&book=CaF2&page=Mali... Random choice below but glass tends to be ~25 - 80 https://refractiveindex.info/?shelf=glass&book=HOYA-C&page=E-C3 https://refractiveindex.info/?shelf=glass&book=HOYA-C&page=E...

foobar1962 3y ago

> Fluorite just has low dispersion and therefore a lens made from it has less chromatic aberration than glass. So why not make the entire lens from fluorite? Because pairing an element with one with different refractive index can lower the overall dispersion. Typically you'll see compound lenses are made of pairs of elements where one is positive (convex) and the other negative (concave): instead of making one lens with power of, say, +4, the group is made from one that's +5 and one that's -1 of different type of glass so the refractive indexes cancel-out dispersion and other aberrations.