# Anti-reflective lenses and interference

• joshwarner
In summary, the anti-reflectance effect is achieved by creating a new reflection that interferes destructively with the original reflection, resulting in a zero reflectance. The thickness of the additional layer is crucial in determining the wavelength at which this effect occurs, with a specific thickness being optimal for a certain wavelength. This explains why the reflectance is reduced more at one wavelength than another, and for λ=500nm, the thickness of the coating layer is such that it results in a zero reflectance.
joshwarner
Homework Statement
Anti-reflective coatings and interference
Relevant Equations
While learning about wave interference in anti-reflective lenses, I came across something mentioning these lenses being only efficient for a certain wavelength. I know these lenses appear blue or purple because, when adding the layer, the reflectance for a wavelength over 600nm isn't worth 0 but is for a wavelength around 500nm. Why is that?

How do you think anti reflectance works?

haruspex said:
How do you think anti reflectance works?
To remove the reflection on the glasses, the additional layer creates a new reflection of the light in order for the two waves reflected to interfere destructively when having a phase difference of an odd multiple of π. So using the distance traveled by the two waves, we get, at normal incidence, with w being the width of the coating layer : (k+1/2)λ=2nw+λ/2. However I still don't see how this explains the reflectance being worth 0 for λ=500nm.

joshwarner said:
To remove the reflection on the glasses, the additional layer creates a new reflection of the light in order for the two waves reflected to interfere destructively when having a phase difference of an odd multiple of π. So using the distance traveled by the two waves, we get, at normal incidence, with w being the width of the coating layer : (k+1/2)λ=2nw+λ/2. However I still don't see how this explains the reflectance being worth 0 for λ=500nm.
Now I am not sure what your question is.
In post #1, I thought you were asking why the reflectance is reduced more at one wavelength than another. That should be evident from your equations in post #3; the ideal thickness of the coating depends on the wavelength.
But now you seem to be asking how it manages to achieve zero reflectance, even at the wavelength the thickness is optimised for.
Which is it?

haruspex said:
Now I am not sure what your question is.
In post #1, I thought you were asking why the reflectance is reduced more at one wavelength than another. That should be evident from your equations in post #3; the ideal thickness of the coating depends on the wavelength.
But now you seem to be asking how it manages to achieve zero reflectance, even at the wavelength the thickness is optimised for.
Which is it?
Sorry for the confusion, I was asking why it was specifically for λ=500nm, I understand how it achieves a zero reflectance and why that only applies for a certain wavelength, but I didn't know it was for λ=500nm because of the thickness of the layer. Many thanks.

## 1. What are anti-reflective lenses and how do they work?

Anti-reflective lenses are a type of coating applied to eyeglasses that helps reduce glare and reflections from the surface of the lenses. This is achieved by allowing more light to pass through the lenses, rather than being reflected off of them.

## 2. What are the benefits of using anti-reflective lenses?

Anti-reflective lenses can improve vision clarity, reduce eye strain, and enhance the appearance of eyeglasses by eliminating distracting reflections. They also allow more light to enter the eye, which can be particularly helpful for those with low light sensitivity or those who spend a lot of time in front of a computer screen.

## 3. How are anti-reflective lenses different from other types of coatings?

Anti-reflective lenses are specifically designed to reduce reflections and glare, while other coatings may serve different purposes such as scratch resistance or UV protection. Anti-reflective coatings are also typically applied to both sides of the lenses, while other coatings may only be applied to one side.

## 4. Can anti-reflective lenses be added to any type of eyeglass lens?

Yes, anti-reflective coatings can be applied to most types of eyeglass lenses, including single vision, bifocals, and progressive lenses. They can also be applied to both plastic and glass lenses.

## 5. Are there any downsides to using anti-reflective lenses?

Some people may experience a slight decrease in contrast or color perception when using anti-reflective lenses. Additionally, the coating may require more frequent cleaning to maintain its effectiveness. However, for most people, the benefits of using anti-reflective lenses outweigh any potential downsides.

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