Newton’s Rings
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.
More from Optics & Light
Other simulators in this category — or see all 44.
Anti-Reflection Coating
Air–film–glass at normal incidence: R(λ) from two-interface interference; quarter-wave design.
Michelson Interferometer
I(Δ) = V cos²(πΔ/λ); tilt fringes; coherence length envelope.
Mach–Zehnder Interferometer
Two-beam recombination; I ∝ cos²(πΔ/λ) vs one-arm OPD; schematic + fringe plot.
Sagnac (Ring) Interferometer
Δφ ∝ Ω·A/λ for counter-propagating beams in a rotating loop — optical gyro idea.
Brewster Angle
tan θ_B = n₂/n₁; R_p→0; θᵢ+θₜ=90°; Fresnel R_s, R_p vs θᵢ.
Fermat's Principle
OPL = n₁AP+n₂PB vs hit point; minimum = Snell path.