Posted by **Marissa** on Tuesday, April 7, 2009 at 1:25am.

So I'm having some trouble with this problem.. Is there an equation or is it just a ratio? I'm having so many problems with this one!

THANKS!

What is the minimum thickness of coating which should be placed on a lens in order to minimize reflection of 531 nm light? The index of refraction of the coating material is 1.32 and the index of the glass is 1.57.

- Physics -
**drwls**, Tuesday, April 7, 2009 at 2:30am
Read "single layer reflection coatings" at

http://en.wikipedia.org/wiki/Anti-reflective_coating#Single-layer_interference_coatings

You need 1/4 wavelength thickness at a refractive index of 1.32

At 531 nm and index 1.32, the wavelength of light is 531/1.32 = 412 nm

The coating thickness should be 1/4 of that

- Physics -
**Marissa**, Tuesday, April 7, 2009 at 12:30pm
that's what i thought, too. but it's not.

2n(material)t= m*(wavelength/2)

going from a medium with higher n to lower n, you divide the wavelength by 2.

thanks for the help, though. i eventually figured it out!

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