A lens of refractive index n is put in a liquid of refractive index n^` of focal length of lens in air is f, its focal length in liquid will be

• A

  $-\frac{{\mathrm{fn}}^{\mathrm{\prime }}(\mathrm{n}-1)}{{\mathrm{n}}^{\mathrm{\prime }}-\mathrm{n}}$

• B

  $-\frac{\mathrm{f}\left({\mathrm{n}}^{\mathrm{\prime }}-\mathrm{n}\right)}{{\mathrm{n}}^{\mathrm{\prime }}(\mathrm{n}-1)}$

• C

  $-\frac{{\mathrm{n}}^{\mathrm{\prime }}(\mathrm{n}-1)}{\mathrm{f}\left({\mathrm{n}}^{\mathrm{\prime }}-\mathrm{n}\right)}$

• D

  $\frac{{\mathrm{fn}}^{\mathrm{\prime }}\mathrm{n}}{\mathrm{n}-{\mathrm{n}}^{\mathrm{\prime }}}$