# Wavelength nature of light: magnesium Fluoride

#### Dx

Hi!

In terms of magnesium fluoride what is the minimum thickness of mag fl coating that must be applied to a glass lens to make it non-reflecting for that wavelength? (the index of refraction of magnesium fluoride is intermediate to that of air and glass)

1/2 wavelength out of phase to eliminate reflection. why is this wrong? I usd the formula t = [lamb] / 4n. I used the minimum thickness (m=0) since the destructive interference will occur over the widest angle. Plz help? Or it this a trick question and there is no minimum thickness, huh?
Dx

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#### Tom Mattson

Staff Emeritus
Gold Member
Originally posted by Dx
I used the minimum thickness (m=0) since the destructive interference will occur over the widest angle.
Obviously, they mean the minimum nonzero thickness. "Zero thickness" means that there is no film at all.

#### Dx

Re: Re: Wavelength nature of light: magnesium Fluoride

Originally posted by Tom
Obviously, they mean the minimum nonzero thickness. "Zero thickness" means that there is no film at all.
Its not that obvious Tom since thats not one of the answers. I understand what your saying so I need to figure out the different wavelengths then, huh?
Dx

#### drnikitin

Originally posted by Dx
why is this wrong? I usd the formula t = [lamb] / 4n. I used the minimum thickness (m=0)
Dx [/B]
You are right. The minimum thickness of the coating is given by t=[lamb]/4n. I think you did an error in calculations. Check it please.

#### Tom Mattson

Staff Emeritus
Gold Member
Originally posted by Dx
I usd the formula t = [lamb] / 4n. I used the minimum thickness (m=0) since the destructive interference will occur over the widest angle.
OK, sorry. I didn't know what m was, and I thought you chose the thickness to be zero.

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