Section 2 - Ok, the we use lenses – but why not air spaced ones?This was the more difficult part of our study, because when we compare oil spaced lenses with air spaced lenses, there are some advantages for both rivaling lens types, that the other type does not have. So, the game is much more balanced here than it was in the case of the "mirrors v.s. lenses" fight. :-)The first thing to notice is the fact, that you may not think about an oil spaced lens as an air spaced lens filled with some kind of fluid. Air spaced lenses feature different curvature radiuses at the surfaces surrounding the air space, and the air space is rather wide, usually wider than 0.1mm. | |
| On the contrary, oil spaced lenses have exactly the same curvature radiuses surrounding the oil, and the thickness of the oil layer is extremely small, usually less than 0.001mm. Usually there is only one single drop of oil between the surfaces, not more. This extremely thin layer of liquid behaves very differently from liquids in macroscopic quantities, e.g. it does not have even the slightest intention to move by the gravitational forces, as gravity has extremely little effect on this very small mass of liquid, but the little fluid has a comparably huge surface, which is tightly connected to the glass surfaces by the cohesion forces between the glass and the oil. So, the oil can not pour out, because there is a much stronger force keeping it between the surfaces than the gravitational force, which is trying to move it out of the lens. Also, the oil can not dry out easily, because through this extremely narrow opening, every oil moleculas takes an "eternity" to get out of the lens, and as soon as a part of the surface would become dry between the lenses, due to the cohesion forces, this area would be filled again with oil from the nearest area automatically (because the cohesion force between the oil and the glass is stronger than it is between two oil moleculas). |
| So it would take many decades for the oil to escape even if the lens edges would not be sealed at all. According to experiments, it is really NOT necessary at all to seal the lens edges, many telescopes were used for a long time without any sealing. But naturally, just to be sure, every manufacturer seals the lens edges, just "to add for double safety". | |
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