A film of soapy water on top of a plastic cutting board has a thickness of 255 nm. What wavelength and color is most strongly reflected if it is illuminated perpendicular to its surface?

the reflected wavelength is lano = 4.55 10⁻⁷ m which corresponds to the blue color

Explanation:

This is a case of reflection interference, we must be careful

* There is a 180º phase change when light passes from the air to the soap film (n = 1,339), but there is no phase change when passing from the pomp to the plastic (n = 1.3)

* the wavelength within the film is modulated by the refractive index

     λₙ =  λ₀ / n

if we consider these relationships the condition for constructive interference is

     2 t = (m + ½)  λₙ

     2t = (m + ½)  λ₀ / n

      λ₀ = 2t n / (m + ½)

we substitute the values

      λ₀= 2 255 10⁻⁹ 1,339 / (m + ½)

      λ₀  = 6.829 10⁻⁷ (m + ½)

let's calculate the wavelength for various interference orders

m = 0

       λ₀ = 6.829 10⁻⁷/ ( 0 +   ½ )

       λ₀ = 13.6 10⁻⁷

       it is not visible

m = 1

      λ₀ = 6,829 10⁻⁷/ (1 + ½)

       λ₀ = 4.55 10⁻⁷

      color blue

m = 2

      λ₀ = 6.829 10⁻⁷ / (2 + ½)

      λ₀ = 2,7 10⁻⁷

   it is not visible

therefore the reflected wavelength is lano = 4.55 10⁻⁷ m which corresponds to the blue color

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