Home Forums Hunting & Shooting Ask The Gunwriters Mule Deer - Optics Question

I recently had an opportunity to read your book on optics for the hunter, and found it to be very informative and entertaining. It is rare to read anything about optics in the sporting press that goes beyond the pamphlets packed with binoculars or scopes.

I do have one question regarding anti-reflective coatings. In the book, you state that the coating thickness is one-half the wavelength. I was always of the impression that it should be one-quarter of the wavelength so that the light reflected from the glass surface gets back to the coating surface 180 degrees out of phase with the light reflected from the coating surface. I am assuming that the index of refraction of the coating is intermediate between that of air and glass (It would do no good otherwise, right?)

Well, the answer to your question is yes, 1/4. However, which "wavelength" are you referring to?

Roy

Strictly speaking, it only works for one wavelength if the light hits at normal (perpendicular) incidence. In reality, because the coating thickness has to vary some, and because light hits the lens off-axis it probably works to some degree for a band of wavelengths. I understand the nominal wavelength is chosen from the middle of the spectrum - yellow/green - somewhere around 550 nm where human vision is most sensitive. Both the red and the blue ends are reflected to a greater degree, which is why many coated lenses look purple.

I always supposed that multicoating was a way of extending the antireflective properties to a broader range of wavelengths, but I don't know how it works.

To be fair, there are so many different substances used these days that I'm not sure that the 1/4 wavelength rule even applies. When this was discovered back in the early 40's, the only substance used was magnesium fluoride. Today, there are so many different substances used in combination and in different thicknesses that they no longer have to target the middle of the visible spectrum as a compromise. The reason you see "550nm" advertised so often is not necessarily because of the compromise being made, but rather where the human eye is most sensitive in daylight hours. If manufacturers would be more ingenuous, we would be seeing less advertising that states, "95%" light transmission. Explaining how coatings work is much easier using charts and diagrams than without. If you are interested you can go to the Georgia State University website or just "google" hyper-physics and follow with "light and vision". They have the most info in a format that's fairly easy to digest.

Roy

Sorry I missed this one earlier.

From talking to various engineers, the way I understand that coating works is that the light-waves pass through the outer coating, and then either go through the glass or are reflected off it. It's these reflected waves that then are "caught" by the inside surface of the coating, and reflected back through the glass again. Without any coating they'd simply be reflected.

Exactly how thick the coating must be depends on the light waves, as the waves of different colors are somewhat different lengths. This is why good multi-coating "traps" even more light within the system, and sends it through the glass to our eye.