To determine the refractive index of a glass slab using a travelling microscope, the minimum number of readings required is:

$A$ glass slab of thickness $3 \, cm$ and refractive index $3/2$ is placed on an ink mark on a piece of paper. The mark is viewed from a distance of $5.0 \, cm$ above the top surface of the slab. At what distance (in $cm$) will the mark appear to be?

$A$ glass slab has a thickness $t$ and a refractive index $\mu$. Calculate the time taken by light to travel through this thickness.

$AB$ is an object of length $l$ dipped in water at a depth of $d$. It is viewed from air at near-normal incidence. The length of the image is

Two parallel rays of red and violet colour pass through a glass slab. Which of the following is correct?

$A$ beaker containing liquid is placed on a table,underneath a microscope which can be moved along a vertical scale. The microscope is focused,through the liquid,onto a mark on the table when the reading on the scale is $a$. It is next focused on the upper surface of the liquid and the reading is $b$. More liquid is added and the observations are repeated,the corresponding readings are $c$ and $d$. The refractive index of the liquid is