Question

# A soap film (n=1.33) is 785 nm thick. White light strikes the film at normal incidence....

A soap film (n=1.33) is 785 nm thick. White light strikes the film at normal incidence. What visible wavelengths will be constructively reflected if the film is surrounded by air on both sides? If there is more than one answer, separate them by a comma.

For constructive interference, the formula to apply is

2nt = (m + .5)λ

λ =2nt / (m + .5)

this is trial and error question

try with m = 0 so

2*1.33*785 =0.5λ

λ = 2*1.33*785/0.5 =4176.2 nm not in visible range

try with m = 1 so

λ = 2*1.33*785/1.5 =1392.06667 nm not in visible range

try with m = 2 so

λ = 2*1.33*785/2.5 =835.24 nm not in visible range

try with m =3 so

λ = 2*1.33*785/3.5 =596.6 nm in visible range

try with m =4 so

λ = 2*1.33*785/4.5 =464.022222   nm in visible range

try with m =5 so

λ = 2*1.33*785/5.5 =379.654545   nm not in visible range

596.6 nm , 464.022222   nm

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