Optical flat
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An optical flat is an optical-grade piece of glass lapped and polished to be extremely flat on one or both sides, usually within a few tens of nanometres. They are used with a monochromatic light to determine the flatness of other surfaces, by means of wave interference.
Optical flats in case. About 2.5 centimetres (1 in) in diameter. The third flat from the left is standing on edge, showing the thickness.
A λ/20 optical flat that has been coated with aluminum, making a first-surface mirror
Two optical flats tested using 589 nm laser-light. At 2 inches (5.1 cm) in diameter and 0.5 inches (13 mm) thick, both surfaces are flat to within 1/10 of the wavelength of the light (58.9 nm), as indicated by the perfectly straight fringes.
Testing the flatness of surfaces with optical flats. The lefthand surface is flat; the righthand surface is astigmatic, with curvatures in two orthogonal directions.
Wave interference
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In physics, interference is a phenomenon in which two coherent waves are combined by adding their intensities or displacements with due consideration for their phase difference. The resultant wave may have greater intensity or lower amplitude if the two waves are in phase or out of phase, respectively.
Interference effects can be observed with all types of waves, for example, light, radio, acoustic, surface water waves, gravity waves, or matter waves as well as in loudspeakers as electrical waves.
Photograph of 1.5cm x 1cm region of soap film under white light. Varying film thickness and viewing geometry determine which colours undergo constructive or destructive interference. Small bubbles significantly affect surrounding film thickness.
Interference fringes in overlapping plane waves
Smartphone with iridescent back panel
An oil spill