Question

A soap bubble 250nm thick illuminated by white light. The refractive index of soap is 1.36.which colours are not seen(destructive), which colours are seen (constructive) in reflected light?

Answer 1

Explanation:

Solution: For destructive interference, we must have $n\,t=m\,\lambda/2$. Thus, the wavelengths that are not reflected satisfy

\begin{displaymath}

\lambda_m = \frac{2\,n\,t}{m},

\end{displaymath}

where $m=1,2,3,\cdots$. It follows that

\begin{displaymath}

\lambda_1 = \frac{(2)\,(1.36)\,(250\times 10^{-9})}{(1)} = 680\,{\rm nm},

\end{displaymath}

and

\begin{displaymath}

\lambda_2 = \frac{(2)\,(1.36)\,(250\times 10^{-9})}{(2)} = 340\,{\rm nm}.

\end{displaymath}