Newtons Rings - Window Cleaners Glossary

Every window cleaner gets to see this at some point in their career.

Newton's rings: The phenomenon of Newton's rings, named after Isaac Newton, who first studied them in 1717, is an interference pattern caused by the reflection of light between two surfaces – a spherical surface and an adjacent flat surface. When viewed with monochromatic light, Newton's rings appear as a series of concentric, alternating bright and dark rings centered at the point of contact between the two surfaces. When viewed with white light, it forms a concentric-ring pattern of rainbow colors, because the different wavelengths of light interfere at different thicknesses of the air layer between the surfaces. 

Newton's rings in a thin film of diesel oil on a wet road, seen in white light.

Newton's Rings, a pattern of light and dark circles visible when a convex lens is placed, curved side down, on top of a flat piece of glass. The pattern was first observed by Sir Isaac Newton. The rings are caused by interference of light waves. When a beam of light is directed downward onto the two pieces of glass, two overlapping beams of light are formed—one from light reflected by the lower surface of the curved glass and the other from light reflected by the upper surface of the flat glass. The light reflected from the flat glass travels farther than the light reflected from the curved glass. Depending on the distance between the two surfaces, light waves in the two beams may be in phase, reinforcing each other, or they may be out of phase, canceling each other out. Because the distances between the two reflecting surfaces increases with distance from the point where the lens and flat glass make contact, the areas where the waves are in phase and out of phase occur in concentric bands around the center of the lens. If the beam of light directed at the two pieces of glass is monochromatic (of a single wavelength), the rings are thin circles of a single color. If white light (which consists of light of many different wavelengths) is used, the rings are fewer in number, but highly colored.

Newtons Rings, a Prism Effect Caused by Varying Thickness of Air Between Two Pieces of Glass.

Anti-Newton Ring Glass: AN (Anti-Newton) glass first came into wide spread use in the late 1960’s.  Enlarger manufacturers were desperate to find a solution of Newton rings for their professional clients.  Photo labs were creating wall size prints and needed to sandwich film between glass to ensure overall print sharpness. Unfortunately Newton Rings were causing high paper wastage.  The Newton rings were being enlarged along with the negative and ruining images. Labs couldn’t use fluid mounting due to heavy use of negative retouching, and glassless carriers allowed a small amount of film curl at the edges causing soft focus in those areas of the print. 

There are no records on file crediting any specific person or company with the invention of AN glass for photographic purposes.  But, with some experimentation, photo labs found it effective and a relatively inexpensive method of solving the Newton Ring problem. AN glass can have its problems depending on how it’s used. The most vexing problem with using any glass sandwich and film is cleanliness.  Dirty environments, poor film cleaning and glass cleaning techniques cause dust problems.  In any glass/film sandwich you have to clean 6 surfaces.  But, benefits far out way the problems.  Dust spots can be retouched from prints and files, Newton rings cannot.

Newton Rings... Optical interference colors created by two pieces of glass pressed snugly together.