If you've ever seen strange concentric ring patterns show up when two panes of glass are pushed together, you've seen Newton's rings. Most people see them as evidence of the wave nature of light. Newton, however, did not.

You've most likely made a set of Newton's rings by accident. Perhaps you've set down a lens from your glasses on a reflective surface. Maybe you've put two pieces of glass on top of each other and one was slightly curved. Whatever happened, you probably noticed the light and dark bulls-eye pattern formed by the two pieces of glass. Those are Newton's Rings.


Light rebounds from both the exterior and interior of a surface. We're used to the fact that when light moves from the air to a piece of glass, that transition reflects some of the light back. When light moves from the glass to the air, that transition also reflects back some light. Although different curves can form a ring pattern, to form Newton's Rings, you generally put a convex piece of glass face-down on a flat piece of glass. The light coming to your eye will come from two sources, the light bouncing back off the interior of the convex glass - off the "bottom of the bowl" - and the light bouncing back from the top of the flat piece of glass. Think of those two bits of light coming back as two waves.

If the top piece of glass were straight, the two waves coming back to your eye would always be a set distance from each other. Sometimes the peaks would always line up, and the reflection would be bright. Sometimes the peaks of one wave would always line up with the troughs of another, and cancel each other out. Then the reflection would be dark. As long as the pieces of glass are straight and parallel, the reflection would always be consistent. Since the convex piece of glass curves, the distance between the two waves changes. At some distances the peaks line up, and at some distances they are canceled out by the troughs. This cancellation and amplification happen over and over, forming rings.

Or that's what we say. The wave model of light so perfectly explains this, that it seems like excellent evidence that light is a wave. Newton, however, saw the light, even that light in Newton's rings, as discrete particles. Of course, he wasn't wrong. Since his time, the model of light being both a particle and a wave has taken over. But the wave model is the most elegant, and obvious, way to explain in Newton's rings. So. Did Newton get Newton's rings wrong?


Via The University of Illinois, School Physics.

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