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20 August, 20:23

Consider a 400 nm thick film of water (n-1.33) on plastic (-1.9). Visible light is incident. on the film firom air. (25 pts) Sketch this situation and clearly indicate the rays which are interfering with one another. (5 pts) a b. Find three wavelengths that will be reflected brightly from the film. (14 pts) c. Find three wavelengths that will be reflected very dimly, if at all, from the film.

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  1. 20 August, 23:05
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    For constructive interference in thin films by reflected light, the condition is

    Path difference = 2μd = odd multiple of λ / 2

    Where μ is refractive index of water, d is thickness of water medium and λ

    is wavelength of light

    λ = 4μd or 4μd / 3 or 4μd / 5

    = 4 x 1.33 x 400 nm = 2128 nm

    or 2128 / 3 = 709.33 nm

    or 2128 / 5 = 425.6 nm

    For destructive interference (dim reflection)

    2μd = multiple of λ = λ or 2λ or 3λ

    2 x 1.33 x 400 nm = 1064

    or 1064 x 2 = 2128 nm

    or 1064 x 3 = 3192 nm.
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