I understand the concept of extraordinary partial dispersion now, which although it would seem related to refractive index, it only refers to the proportion of difference of refraction of different wavelengths, whereas refractive index refers to how much the light rays veer away from the normal after it has passed through the medium.
Unless I'm missing something, it's just as transparent as good glass - they're both transparent because there's very few imperfections to absorb or scatter the photons (light photons is redundant ) at the wavelengths of interest.
So in terms of my magic answer for what makes Fluorite transparent,
Fluorite is transparent because the electrons within each atom of Calcium or Fluorine are unable to escape their electron shells and are thus unable to absorb or reflect the incoming photons, meaning that the photons are transmitted.
Fred Lindsey wrote:
If I am correct there, why are glass and fluorite not 100% transmissive.
(Sorry if I'm being dumb now, I'm 16 and haven't had much sleep in the last two days.)
Actually, I'm probably right in saying that by imperfections, you mean that the photons transmitted at the smallest wavelengths are absorbed, resulting in a sub-100% transmission.
Is the reason fluorite is more transmissive than optical glass because its chemical structure allows the transmission of photons with smaller wavelengths?
Fred, you are trying to explain something which you will probably not fully understand for years to come: Colour and transparency relate at least in part to the electronic band structure of the material, which in turn is explained via quantum mechanics. In short though, a transparent material is one which does not absorb light in the visible region of the electromagnetic spectrum. Or to put it another way, the energy of the incoming light does not correspond to any available energy by which the material can enter an excited state via absorption of the light.
Once you have a material which does not absorb visible light you still need some other things to be true for it to be transparent. If it is crystalline it needs to exhibit a low number of crystal flaws, no cracks, and it is implicit that it should be a single crystal. The presence of crystal flaws and cracks scatters the light within the crystal, which causes the otherwise transparent material to become white (grind up some glass to see this effect).
As for the structure. CaF2 is the mineral "Fluorite" and the crystal structure is named after it. It is cubic, not hexagonal. As a point of interest, many other compounds also crystallise with the fluorite structure, not just CaF2.
skibum5 wrote:
the atoms in that arrangement are able toand the structure is very uniform so you get lots of phase reinforcement in the forward direction and don't lose lots of it to scattering
I don't understand what you mean here.
Funnily enough, my teacher has said she finds my arguement that I quoted plausible and will accept it
Can we get onto extraordinary partial dispersion and why the proportions are different.
How does the velocity change when within the fluorite affect the outcome of the dispersed light?
- Why does light disperse?
- Why do transparent substances have a refractive index?
- Why does light disperse in a non-linear fashion when passed through fluorite?
Wow, lots of great info here. My first thought was to suggest hitting up some of the better refracting telescope manufacturer's websites. Just to show Canon's involvement, here's a snippet from the Takahashi website (www.takahashiamerica.com)...
"Finally, in 1977, Takahashi Seisakusho Ltd. of Japan introduced the world's first astronomical telescope with a fluorite objective. By working closely with optical experts at Canon Inc., the technology for making fluorite lenses as large as 150mm (6") in diameter was developed. The remarkable performance of the fluorite element allowed the production of f/8 telescopes with only two elements in the objective. Coating technology had also improved during this period so that the glass elements could be fully multi-coated to prevent light loss and ghosting. The result was an air-spaced apochromatic doublet. Color correction was as good as or even exceeding most triplet systems and contrast was far superior."
wimg wrote:
Fluorite is not hexagonal, it is isometric, which under normal conditions means cubic, although octagonal is common too, and occasionally occurs as dodecahedral shapes (12 surfaces).
Okay, let the chemist jump in here: In CaF2 each Ca2+ ion is surrounded by 8 F- ions at the corners of a cube, and each F- ion by 4 Ca2+ ions at the corners of a regular tetrahedron. Or differently said - the coordination numbers are 8 for Ca2+ and 4 for F- in the fluorite structure.
Canon is not the only lens manufacturer to use Fluorite - Minolta did back in the day, and a few others as well. None marketed it like canon did though.
You should also pick up a glass map - fluorite is on it, and it gives a nice graphical representation of the relationship between index of refraction and dispersion , along with abbe #, etc.
Fred--you're going to really going to need quantum mechanics to comfortably explain dispersion, transparency, etc.
The crystal needs to be very pure with few defect sites, as those serve as both scattering points and transmission losses.
Transparency is basically when there's not enough energy in the photon to be absorbed (as energy levels are quantized, so there's a discrete gap in energy) into a binding electron in order to delocalize the electron. Delocalization, in other words, is effectively breaking that bond (but not really) and allow the binding electron to move freely around the crystal.
I hope some day you end up actually getting enough coursework to understand what people have said. Some really smart explanations have been given to you, but it's way beyond your level. No insult intended, by the way.
) at the wavelengths of interest.