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)

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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