This animation illustrates destructive thin-film interference (which can be applied to anti-reflecting coating). The yellow wave is incident light (in air). When it reaches a substance with a higher refractive index (here n = 1,5), part of the light is reflected transmitted (shown in yellow: the wavelength becomes shorter) and part of the light if reflected with a 180° shift(shown in green). If the third medium (at the bottom) has a refractive index higher than 1.5, part of the transmitted light is reflected with a 180° shift (shown in pink). The reflected waves cancel each other (destructive interference) if the thin film has a proper thickness.
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