A certain region of a soap bubble reflects red light of vacuum wavelength `lambda = 650nm`. What is the minimum thickness that this region of the soap bubble could be have? Take the index of reflection of the soap film to be 1.41.

Options

A. `1.2 xx 10^(-7) m`
B. `650 xx 10^(-9) m`
C. `120 xx 10^(7) m`
D. `650 xx 10^(5) m`

Answer

a

Explanation

There is air on both sides of the soap film. Therefore, the reflections of the light produces a net `108^(@)` phase shift.
The condition for bright fringe is
`2 t = (m + (1)/(2)) lambda_("film")`
`t = ((m + (1)/(2)) lambda_("film"))/(2) = ((m + (1)/(2)) lambda)/(2n)`
`= ((1)/(2) (650 xx 10^(-9)))/(2(1.41)) = 1.2 xx 10^(-7) m`