Question

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 have? Take the index of reflection of the soap film to be $1.41$

• A. $1.2\times {10}^{-7}m$
• B. $650\times {10}^{-9}m$
• C. $120\times {10}^{7}m$
• D. $650\times {10}^{5}m$

Answer 1

The correct option is A $1.2\times {10}^{-7}m$

By equation of destructive interference,
thickness, $t=\frac{1}{2}\frac{(m+0.5)\lambda }{1.41}$
where $m$ is any non-negative integer,
$\lambda =650\ast {10}^{-9}$.
For minimum thickness, take $m=0$
So, $t=1.152\ast {10}^{-7}\approx 1.2\ast {10}^{-7}$