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Fizeau’s Fringes

Fizeau’s Fringes

Fizeau’s fringes are fringes of equal thickness produced by the interference of light reflected from the two surfaces of a thin wedge-shaped film.

  • They are generally straight, parallel, and equally spaced.
  • They are localized near the film.
  • The thickness of the film varies from point to point.
  • The optical path difference depends on the thickness of the film.

For a film of refractive index μ, thickness t, and refracted angle r:

Δ = 2μt cos r

For a wedge-shaped air film at nearly normal incidence:

Δ = 2t

The condition for dark fringes in reflected light is:

2t = mλ

If the wedge angle is θ, then:

t = xθ

Therefore:

2xθ = mλ

The fringe width is:

β = λ / (2θ)

Difference Between Haidinger’s and Fizeau’s Fringes

Property Haidinger’s Fringes Fizeau’s Fringes
Type Fringes of equal inclination Fringes of equal thickness
Film Plane-parallel film Wedge-shaped film
Shape Circular Circular, Straight and parallel
Localization At infinity Near the film
Main variable Angle of inclination Film thickness
Typical example Plane-parallel plate Air wedge
Fringe spacing Generally not uniform Uniform for a uniform wedge