Plane wave fronts are incident on a glass slab which has refractive index as a function of distance z, according to the relation  ${\text{μ=μ}}_{\text{0}}^{ }\left({\text{1-z}}^{\text{2}}{\text{/z}}_{\text{0}}^{\text{2}}\right)$, where ${\text{μ}}_{\text{0}}$ is the refractive index along the axis and $z_0$ is a constant. This glass slab can act as lens of focal length f. By using the concept of optical path length calculate the focal length of the slab. Consider t to be very small as compared to f.