The previous article introduced the concept of optical interference , which is the ability of light waves to interact with one another. When light waves of similar wavelengths overlap, they can either reinforce one another or cancel one another out, depending on the phase difference between the waves. The two cases are described as follows:
Constructive Interference occurs when light waves interact to create a combined wave with a larger amplitude than the original waves.
Destructive Interference occurs when light waves interact to create a combined wave with a smaller amplitude than the original waves.
Optical interference has many applications. One use that many people see every day (or rather *don’t* see!) is anti-reflection (AR) coatings, which are commonly used to reduce glare from eyeglasses. When light hits the front of the eyeglasses, about
2% 4% of the light reflects away from the lens. Reflection occurs at both the front and the back surface of the lens, so the net effect is that a total of 4% 8% of the light reflects away from the lens. The reflected light obscures the wearer’s face. Furthermore, although 4% light-loss is not really noticeable for someone wearing eyeglasses, for larger optical systems, such as cameras, often have several lenses. Losing 2% 4% of the light at every surface adds up quickly. Furthermore, the unwanted reflections create “stray light,” which can have all sorts of problems.
Unwanted reflections can be minimized by applying (or “coating”) a very thin layer of a transparent material onto each surface of the lens. In optics, these layers of material are called “thin films.” Light reflects from both the top and the bottom of the film, and the two reflecting waves interfere with one another. By carefully controlling the thickness of the film, it is possible to create just enough phase difference between the two waves to cause destructive interference. In the figure, notice that the peaks of the top wave align with the “troughs” of the bottom wave. The two waves cancel one another out, which eliminates the glare from the lens. It also means that more light transmits through the lens.
Pulling all this together, a “very least you need to know” definition of an AR coating is as follows:
An antireflection (or “AR”) coating, is a very thin film of transparent material that minimizes unwanted light reflections by creating destructive interference between waves reflected from the top and bottom of the film.
This type of single-layer AR coating is the simplest form of a more general class of optical coatings called “thin film interference filters,” but that gets outside of “the very least you need to know” for right now.
A big thank-you to Jeffrey Richling for pointing out that 2% is a significant understatement of the amount of light reflected from eyeglasses. I have upped that to 4% in the article.
The amount of light that a surface reflects depends on a feature known as “refractive index.”
I will write about refractive index in an upcoming blog.