What is the time taken (in seconds) for light to cross a glass slab of thickness $4 \ mm$ and refractive index $\mu = 3$?

$A$ beaker contains water up to a height $h_{1}$ and kerosene of height $h_{2}$ above water. The total height of (water $+$ kerosene) is $(h_{1} + h_{2})$. The refractive index of water is $\mu_{1}$ and that of kerosene is $\mu_{2}$. The apparent shift in the position of the bottom of the beaker when viewed from above is:

$A$ microscope is focused on a point object and then its objective is raised through a height of $2 \ cm$. If a glass slab of refractive index $1.5$ is placed over this point object such that it is focused again,the thickness of the glass slab is......$cm$

The lower half of a vessel of depth $2d \text{ cm}$ is filled with a liquid of refractive index $\mu_1$ and the upper half with a liquid of refractive index $\mu_2$. The apparent depth of the vessel seen perpendicularly is

The apparent depth of a needle lying at the bottom of a tank,which is filled with water of refractive index $1.33$ to a height of $12.5\,cm$,is measured by a microscope to be $9.4\,cm$. If water is replaced by a liquid of refractive index $1.5$ up to the same height,what distance would the microscope have to be moved to focus on the needle again (in $,cm$)?

$A$ microscope is focused on an object at the bottom of a bucket. If liquid with refractive index $\mu = \frac{5}{3}$ is poured inside the bucket,then the microscope has to be raised by $30\,cm$ to focus the object again. The height of the liquid in the bucket is $..........\,cm$.