The wavelength of light having wave number $400{\mathrm{cm}}^{-1 }\mathrm{is}$

The relation between wave number and wavelength is

$\overline{\mathrm{\vartheta }}$$=\frac{1}{\mathrm{\lambda }}$

$\Rightarrow \mathrm{\lambda }=\frac{1}{\mathrm{\vartheta }}$

$\mathrm{\lambda }=\frac{1}{4000{\mathrm{cm}}^{-1}}=250\times {10}^{-6}\mathrm{m}$

$\therefore 250\mathrm{\mu m}; \mathrm{since} 1\mathrm{\mu m}={10}^{-6}\mathrm{m}$