Why are the fringes circular?

The lens–plate gap thickness depends only on distance from the contact point (to good approximation), so loci of equal thickness are circles centered on the contact.

What does the π checkbox change?

If only one of the two reflections picks up a π phase (low-to-high index), bright and dark conditions swap. Toggling the box lets you match either textbook convention.

Can Newton’s rings measure lens curvature?

Yes. Measuring ring diameters and knowing λ yields the radius of curvature R of the lens surface via the small-angle fringe spacing relation.

Why does the simulation look pixelated?

It uses a finite grid for speed. High-resolution cameras in the lab see smooth rings until diffraction limits intervene.

Newton’s Rings

Spherical lens on flat glass: air-film thickness fringes; optional π phase on reflection.

About this model

Newton’s rings appear when a slightly curved optical surface (often a plano-convex lens) contacts a flat plate, trapping a thin air film whose thickness grows with radius. Light reflected from the top and bottom of the film interferes. For near-normal viewing the optical path difference is about 2nt, with t the local gap height and n the film index (air ≈ 1). A π phase shift on reflection from the lower-index side of an interface can invert bright/dark conditions; the checkbox models the common case of a phase jump when reflecting from the glass plate. The circular symmetry makes fringes concentric; the small-angle radius of the m-th dark ring scales like √(mλR/n) when the extra π is included. The raster image uses cos(4πnt/λ + optional π). Assumptions: quasi-monochromatic light, scalar interference, no finite coherence or oblique incidence — adequate for qualitative lab demos.

Who it's for: High school and college optics labs studying thin-film interference, phase shifts on reflection, and radius-of-curvature measurements.

Key terms

Newton’s rings
Thin film
Interference
Phase shift on reflection
Optical path difference
Plano-convex lens
Air wedge
Fringe radius

How it works

Concentric interference fringes from a thin air wedge between a spherical lens and flat glass: optical path difference 2nt plus possible π phase on reflection.

Frequently asked questions

Why are the fringes circular?: The lens–plate gap thickness depends only on distance from the contact point (to good approximation), so loci of equal thickness are circles centered on the contact.
What does the π checkbox change?: If only one of the two reflections picks up a π phase (low-to-high index), bright and dark conditions swap. Toggling the box lets you match either textbook convention.
Can Newton’s rings measure lens curvature?: Yes. Measuring ring diameters and knowing λ yields the radius of curvature R of the lens surface via the small-angle fringe spacing relation.
Why does the simulation look pixelated?: It uses a finite grid for speed. High-resolution cameras in the lab see smooth rings until diffraction limits intervene.

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Newton’s Rings

About this model

How it works

Frequently asked questions

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About this model

How it works

Frequently asked questions