A microscope is having objective focal length $1 \mathrm{cm}$ and eye piece of focal length $6 \mathrm{cm}$. If tube length is $30 \mathrm{cm}$ and image is formed at the least distance of distinct vision, what is the magnification produced by the microscope. $($ take $, D=25 \mathrm{cm})$

A glass slab consists of thin uniform layers of progressively decreasing Refractive Indices $RI$ (see figure) such that the $RI$ of any layer is $\mu -m\mathrm{\Delta }\mu$. Here, $\mu$ and $\mathrm{\Delta }\mu$ denote the $RI$ of zeroth layer and the difference in $RI$ between any two consecutive layers, respectively. The integer $m=0,1,2,3,\dots$ denotes the numbers of the successive layers.

A ray of light from the zeroth layer enters the first layer at an angle of incidence of $30°$. After undergoing the $m^{\mathrm{th}}$ refraction, the ray emerges parallel to the interface. If $\mu =1.5$ and $\mathrm{\Delta }\mu =0.015$ the value of $m$ is