Understanding
and Preventing
Newton Rings
Newton rings are a phenomenon that can occur when light reflects between two surfaces: a convex lens and an adjacent flat surface. This effect, named after Sir Isaac Newton who first studied it, results in a series of concentric, alternating bright and dark rings. These rings are caused by the interference of light waves.
Sir Isaac Newton in 1704, in Proposition XII on p. 78 of his “Opticks: A treatise of the reflexions, refractions, inflexions and colours of light” stated:
Reflections and the conditions for constructive and destructive interference
Thin-film interference is a natural phenomenon in which the combination of two or more electromagnetic waveforms forms a resultant wave in which the displacement is either reinforced or canceled.
Newton Rings
Reflections and phase changes at the glass-air interfaces
The sketch above shows the geometry close to the point of contact between the convex lens (top, or a pressed film) and the glass or plastic flat (bottom).
Consider first the arrows closest to the point of contact, where the separation t between interfaces is less than a quarter wavelength, λ/4. Now consider the reflection from the top surface and that from the bottom: the two parallel rays travelling upwards. The component of phase difference between these two, is due to their different pathlengths, which is much less than π.
Now consider the phase difference due to reflections. At the upper surface, the ray reflects going from glass of film towards air – high n to low n – so the phase change is zero, as indicated on the sketch. At the lower surface, the rays reflects going from air towards glass – low n to high – so the phase change is π. Thus, a small phase change due to path length but a π phase change due to the two reflections, these two rays are about π radians or half of one cycle out of phase, which gives destructive interference. Consequence: in this region close to the point of contact, there is destructive interference in reflection. Destructive interference happens when two waves meet and cancel each other out, resulting in a decreased amplitude or complete cancellation. This occurs when the crests of one wave meet the troughs of the other wave.
Next consider the middle set of rays in the sketch. Here the thickness is t = λ/4, so the path difference is 2t = λ/2. This gives a phase difference of π. Add this to the π from the reflections and the phase difference is 2π – the two rays are out of phase by one complete cycle. So, provided the coherence length is sufficiently long, these rays give constructive interference in reflection. When two waves travel in the same direction and are in phase with each other, their amplitude gets added. These waves are said to have undergone constructive interference.
Finally, the rays in the sketch at left are for an air film thickness of t = λ/2, so the path difference is 2t = λ. This gives a phase difference of 2π. Add this to the π from the reflections and the phase difference is 3π – the two rays are out of phase by one and a half complete cycles. This time we have destructive interference in reflection.
And so on: our thin film has a varying thickness, so we have different interference conditions at different thicknesses. This leads to the alternating rings of constructive interference (bright rings) and destructive interference (dark), as shown in the photograph below.
The previous sketches and physical descriptions were presented at the UNSW School of Physics, Sydney, Australia.
FORMATION OF NEWTON RINGS
Newton rings are created when a thin air film exists between two surfaces, such as a lens and a glass plate. Here’s a step-by-step breakdown of the process:
1. Thin Film Interference:
When light waves hit the thin air film, they are partially reflected and partially transmitted at both the top and bottom surfaces of the film.
2. Wave Interference:
The reflected light waves interfere with each other. Depending on the thickness of the air film, this interference can be constructive (resulting in bright rings) or destructive (resulting in dark rings).
3. Variable Thickness:
The thickness of the air film varies radially from the point of contact outward, leading to the appearance of concentric rings.
The varying thickness of the air gap causes different wavelengths of light to interfere at different points, producing the characteristic pattern of Newton rings.
PROBLEMS CAUSED BY NEWTON RINGS
In various optical display applications, Newton rings can be problematic. They can obscure details, reduce image clarity, and create unwanted artifacts. For example, in photographic film scanning, Newton rings can appear as unwanted patterns that distort the scanned image.
PREVENTING NEWTON RINGS
Anti-Newton Ring Coatings:
To mitigate the issues caused by Newton rings, anti-Newton ring (ANR) coatings are used. These coatings are designed to prevent the formation of the air film that causes interference.
Microscopic Roughness:
ANR coatings typically have a microscopic roughness that prevents the formation of a uniform air film between the surfaces. The rough surface ensures that any air gaps are too irregular to produce coherent interference patterns.
Reduced Reflection:
Some ANR coatings also reduce reflections by using materials that minimize the amount of light reflected back into the lens. This reduces the chances of interference patterns forming.
Newton Rings are an annoying and unwanted phenomenon, that must be properly addressed when assembling or manufacturing screens, lenses or frames.
Newton rings are an optical interference pattern caused by the reflection of light between two surfaces with a thin air film between them. While visually interesting, they can cause significant issues in various optical applications. Anti-Newton ring coatings prevent this phenomenon by introducing microscopic roughness, reducing reflections, and matching optical indices, ensuring clear and accurate imaging without unwanted interference patterns.