A parallel beam of light travelling in x direction is incident on a glass slab of thickness t. The refractive index of the slab changes with y as `mu = mu_(0) ( 1 – ( y2) /( y_(2)^( 0) ) ) `where `mu_(0)` is the refractive index along x axis and `y_(0)` is a constant. The light beam gets focused at a point F on the x axis. By using the concept of optical path length calculate the focal length f. Assume` f gtgt t` and consider y to be small.

Answer

`(y_(0)^(2))/(2mu_(0)t)`