Question

A light wave enters from air into a medium of refractive index $\frac{4}{3}$. If wavelength of light in air is $6000\mathring{A}$, then the wave number of light in the medium is:

• A. $1.1\times {10}^6$
• B. $2.2\times {10}^6$
• C. $4.4\times {10}^6$
• D. $6\times {10}^6$

Answer 1

The correct option is B $2.2\times {10}^6$

Wave number is reciprocal of wavelength.
Wave number is given by
$\overline{v}=\frac{1}{{\lambda }^{\prime }}=\frac{\mu }{\lambda }$ (As, ${\lambda }^{\prime }=\frac{\lambda }{\mu }$)
$\Rightarrow \overline{v}=\frac{\frac{4}{3}}{6000\times {10}^{-10}}$
$=\frac{4\times {10}^7}{3\times 6}=2.2\times {10}^6/m$