What Causes Newton Rings - A Simple Explanation

There's been a lot of misinformation and confusion out there about what causes newton rings on graded comic book cases.

Let's start with the basics and keep it REAL simple because optical equations can get complicated very quickly and I'll lose most of you.

Consider a convex lens placed on top of a flat piece of glass. Both are highly polished.



R = the radius of curvature of the lens
r = the radius from the center of contact
t = the thickness of the thin film of air between the two surfaces

For the purposes of this analysis (to keep it simple) we are only going to consider r and t. As r is increased, t increases.

Now take a look at the photo of the interference pattern on the right. The dark area at the center is the point where the convex lens surface touches the flat piece of glass below it. In this area, the thin film of air between the lens and the flat piece of glass has a thickness much less than the wavelength of light. This is surrounded by bright and dark circles, due to constructive and destructive interference respectively.

Remember, as r increases, t increases. In other words, as we move outward from the point of contact, the thickness of the thin film of air increases. This, in turn, causes varying interference conditions, which leads to the alternating rings of constructive interference (bright rings) and destructive interference (dark rings), as shown in the photograph.

Ok, so that's the physics behind the optical phenomenon known as newton rings. Let's move on and apply this to the newton rings seen on graded comic book holders.

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Newton rings on graded comic book holders are caused by contact and pressure between two layers of transparent plastic.

This is graphically illustrated in the video below of the Creep Engine Case which CGC used prior to the current case (officially 4/4/16 to 6/24/16).  This bold design utilized pressure to hold the comic book in place, thereby reducing or eliminating Shaken Comic Syndrome (SCS).  The outer case applied pressure to the comic book within which was wrapped in archival-quality Mylar sheets.  Unfortunately, very prominent newton rings were formed because of the high pressure contact between the outer case and the Mylar sheets. 

To see hundreds of examples of this, click HERE.

In the video, you will also see how these prominent newton rings were used to created a pressure map.  This map highlighted the pressure non-uniformity of the case.  And, of course, this non-uniformity is what caused the creep-engine effect, which ultimately caused waviness of the comic books within the case.



The new (modified) CGC case and the cases prior to the Creep Engine Case do not use pressure to hold the comic book in place, they use an inner well/sleeve.  Newton rings on graded comic book holders using this design are caused by contact and pressure between the (outer surface) of the inner well/sleeve and the (inner surface) of the outer case.  The video below illustrates this process.  In it, I am creating newton rings on the front of the holder by applying a slight point pressure to the back of the holder which compresses the surfaces.



So, as you can see, newton rings on graded comic book cases are caused by contact and pressure between the two layers of transparent plastic. Contact alone is not usually enough since plastic is not a highly polished surface and has some level of surface roughness which increases t (remember that t = the thickness of the thin film of air between the two surfaces) to a point beyond the wavelength required for newton rings to appear. Pressure creates a more intimate contact and decreases t.