Okay, I bought a new pair of glasses the other day and I asked for an anti-reflective coating on the lenses. It’s nice and all, but how does it work? What gives it the colour, and why is it so hard to clean?
``You’re just an empty cage girl if you kill the bird.’’ – Tori Amos.
I can answer the how does it work part.
Imagine you’re a light ray. When you hit the lens of a normal pair of glassed, you are going to go through. While doing so, you will refract (bend). If you bend enough, you reflect. This amount of bending is measured by the angle from the normal, and is called the critical angle. The critical angle is dependant on the medium through which you travel.
The antireflective coating makes it change mediums twice. The light bends going through it, but not as much as in glass. It then bends again when it hit the glass, but not as much as in the first case because the difference in the index of refraction (the amount light bends in a medium) is smaller. So it is much less likely to reflect.
I appologize ahead of time for any small errors, I havent done optics for 5 years. But it should be more or less correct.
He who is truly wise is the one who knows how much he has yet to learn.
I think that AuMatar might have part of the explanation, but most reflection from glass is ordinarily from the front surface, not the back, which it would need to be for what he’s descibing. Most anti-reflective coatings, IIRC, work on the principle of destructive interference. If you put a very thin layer of transparent material on the front of the glasses, then the light that reflects off of the front surface and the light that reflects off of the interface between coating and glass can exactly cancel each other out.
“There are only two things that are infinite: The Universe, and human stupidity-- and I’m not sure about the Universe”
–A. Einstein
Chronos is correct, it is interference. The magic is in the thickness of the coating, which is set to 1/4 the wavelength of visible light. Some light reflects from the front surface of the coating, and some from the rear surface (i.e. the interface between coating and glass). Because the coating is 1/4 of the wavelength thick, the phase of the two reflected light is exactly opposite, and the peaks of the one cancel out the troughs of the other. This dramatically reduces the reflection. However, visible light contains a wide range of colors, so the anti-reflective coating works perfectly for one color and less perfectly for all other colors. That’s why you get colored reflections.
I’m not sure why they’re hard to clean. They are extremely thin films though, and more fragile than the glass underneath. Also, the reflection from the two surfaces must be reasonably similar for the cancellation to work, so they are limited in the kinds of material they can use.
By the way, if you instaed have a 1/2 wavelength coating, you maximize the reflection instead. They do this on mirrors, like those on telescopes. If you have many layers of different materials (well, alternate between two materials) each with 1/2 wavelengh thickness, you get even more reflection. With enough such layers you can reflect anything - even X-rays. Multilayer X-ray optics is a hot topic these days, because semiconductor manufacturers are interested in using it for lithography. X-rays have shorter wavelengths than visible light and makes it possible to create finer structures.
Damn it, sounded good. Well, I’ll bow to superior knowledge on this one, although I’ll hold on that my answer may be partly correct. prepares to get shot down on that too