Step-by-step explanation:
Formula for path difference is as follows.
x = 2tn
and, refractive index (n) =

Thickness is calculated as follows.
Thickness (t) =

Area =

=

=

=
m
Now, the refractive index will be calculated as follows.
For drop, n =

For B drop, n =

So, n =

=

= 1.5
Thus, we can conclude that index of refraction of the oil is 1.5.