A soap film has a refractive index of 1.40. How thick must the film be, if it appears black, when mercury light of wavelength 546.1 nm falls on it normally? (1.95 x 10⁻⁷ m) 



Given:

Refractive Index = n = 1.4 

Number of  fringe = m =  1

Wavelength of light = λ = = 546.1 nm = 546.1 ｘ 10⁻⁹ m


To Find:

The thickness of the film = t =  ?


Solution:


The formula for the thickness t is

t = `\frac {mλ}{2n}`


by putting values

t = `\frac {1 ｘ 546.1 ｘ 10⁻⁹ m}{2 ｘ 1.4}`

t = 1.95 ï½˜ 10⁻⁹ m ----------------Ans.


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