Multiple surfaces When light makes multiple reflections between two or more parallel surfaces, the multiple beams of light generally interfere with one another, resulting in net transmission and reflection amplitudes that depend on the light's wavelength. The interference, however, is seen only when the surfaces are at distances comparable to or smaller than the light's coherence length, which for ordinary white light is few micrometers; it can be much larger for light from a laser.
multiplE surfacEs wHEn liGHt MAkES MuLtiple ReflECTions bETweEn TWo or moRe paRallel Surfaces, the mULtIPle beAms OF Light genErallY iNtErfeRE WIth oNe AnotHer, rEsuLting iN NeT tRAnsmisSion AnD rEflection amPLitudEs thAT depEnd On the lIGHT'S WaVeleNGTh. the interfeRENCE, HoweVEr, is seen Only whEn ThE SURfAcES aRE at diSTanCeS compaRAble tO Or sMalleR Than thE lIgHt'S cOheReNce lEngTh, wHich foR OrdinaRy white lIght Is fEw mIcrometeRs; iT cAn be Much larger fOR lIghT frOm A LASeR.