Newton's rings
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Circular fringes from a lens on a flat surface.
A convex lens on a flat plate produces concentric rings from an air-wedge interference; the radius of the nth dark ring is r_n = √(nλR). Used to test lens surfaces.
R = 1 m, λ = 589 nm: r₁₀ = 2.4 mm.
Centre is dark due to the phase change.
In more detail
Newton's rings are concentric interference fringes formed between a convex lens and a flat glass plate. The air gap thickness varies with radius, producing rings of alternate light and dark.
Real-world uses
- Newton's rings are used to test the flatness of optical surfaces.
- Lens quality checks use interference patterns.
- Opticians test the flatness of lenses and mirrors by looking at the circular fringes formed between them.
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Frequently asked questions
Why are the rings concentric?
Because the air gap thickness depends only on distance from the centre.
What does a dark centre mean?
Contact between the lens and plate causes a phase change, making the centre dark.
What do the rings show?
The ring spacing depends on the air gap, so irregular rings reveal a surface that is not flat.
Related topics in Optics
Fringe width · Intensity in interference · Thin film interference · Michelson interferometer · Single slit diffraction · Diffraction grating