If a thin soap film (n = 1.36)reflects predominately red light (about 680 nm),then what is the minimum thickness of the soap film? A)1.25 $\times$ 10^-7 m B)6.80 $\times$ 10^-7 m C)2.50 $\times$ 10^-7 m D)5.00 $\times$ 10^-7 m E)None of these is correct.

Question

Quizplus · Accepted Answer

For constructive interference in a thin film, the path difference should be an integral multiple of the wavelength in the medium. The minimum thickness for the first order (m=1) constructive interference is given by $t = \frac{\lambda}{2n}$, where $\lambda$ is the wavelength of light in vacuum and $n$ is the refractive index of the film. For red light ($\lambda = 680$ nm) and a soap film with $n = 1.36$, the minimum thickness is $t = \frac{680}{2 \times 1.36} = 250$ nm, which is option A when correctly interpreted as $1.25 \times 10^{-7}$ m or 250 nm.

If a Thin Soap Film (N = 1 ×\times× 10-7 M B)6

If a Thin Soap Film (N = 1 $\times$ 10^-7 M
B)6