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# Re: Magnesium Flouride

• From: John Denker <jsd@AV8N.COM>
• Date: Sat, 14 Feb 2004 17:43:51 -0800

Quoting David Abineri <dabineri@CHOICE.NET>:

In high school optics we frequently do problems involving a thin film of
Magnesium Flouride on a lens to enhance the tranmission in the middle of
the visible spectrum

OK.

because of a lack of sensitivity by the film (I
hope I have this right).

I don't think it has anything to do with the film. It
has to do with the reflection off the index-mismatch.
It's called an AR (anti-reflection) coating.
MgF is convenient because it has an index that is
roughly the geometric mean of the two things (air
and glass) that you're interfacing, and it is
relatively easy to apply.

It's an easy wave-mechanics exercise to calculate the
optimal thickness of the AR coating, given the indexes
of the three materials. Hint: set it up as a wave
problem, not as a bouncing-rays problem.

What about digital cameras?

Same problem, same solution.

Do their lenses need any form of such correction

Yes.

===============

Fancy lenses use *multiple* layers to gradually and progressively
match the indexes. These are called multi-coated lenses.

A single-layer AR coating that is "just right" at a given wavelength
lambda will be as bad as it possibly can be at lambda/2. The
AR coating becomes what I call an "R" coating. This is what
produces the bluish cast you see on mid-quality lenses.
With multiple layers you can flatten out the frequency response.
The engineering of layer thickness etc. is beyond the scope of
high-school physics class.

A good multi-coated lens is quite something to behold: it looks
like it isn't there.