When a ray of light passes from one medium to another,which quantity remains constant?

If the velocity of light in the first medium is $v_1$ and in the second medium is $v_2$,then the refractive index of the second medium with respect to the first medium is given by:

$A$ ray of light passes through four transparent media with refractive indices $\mu_1, \mu_2, \mu_3$ and $\mu_4$ as shown in the figure. The surfaces of all media are parallel. If the emergent ray $CD$ is parallel to the incident ray $AB$,we must have

$A$ monochromatic beam of light is travelling from medium $A$ of refractive index $n_{1}$ to a medium $B$ of refractive index $n_{2}$. In the medium $A$,there are $x$ number of waves in a certain distance. In the medium $B$,there are $y$ number of waves in the same distance. Then,the refractive index of medium $A$ with respect to medium $B$ is

$A$ beam of light is incident on a glass plate at an angle of $60^{\circ}$. The reflected ray is polarized. If the angle of incidence is $45^{\circ}$,then the angle of refraction is:

$A$ light ray passes through four transparent media with refractive indices $\mu_1, \mu_2, \mu_3$ and $\mu_4$ as shown in the figure. All surfaces are parallel to each other. If the emergent ray $CD$ is parallel to the incident ray $AB$,then: