The colors seen on an oil film floating on the surface of water are due to interference. What should be the order of the thickness of the oil film?

In a Fresnel's biprism experiment,the fringe width is $1 \,mm$. The distance of the nearest dark fringe from the central bright fringe is .......$mm$.

Assertion: Newton's rings are formed in the reflected system when the space between the lens and the glass plate is filled with a liquid of refractive index greater than that of glass; the central spot of the pattern is bright.
Reason: This is because the reflection in these cases will be from a denser to rarer medium and the two interfering rays are reflected under similar conditions.

$A$ parallel coherent beam of light falls on a Fresnel biprism of refractive index $\mu$ and refracting angle $\alpha$. The fringe width on a screen at a distance $D$ from the biprism will be (wavelength $= \lambda$).

Imperfections in optical lenses can be observed with the help of

$A$ ray of light of intensity $I$ is incident on a parallel glass slab at a point $A$ as shown in the figure. It undergoes partial reflection and refraction. At each reflection,$25\%$ of the incident energy is reflected. The rays $AB$ and $A^1B^1$ undergo interference. The ratio $I_{\text{max}} / I_{\text{min}}$ is (in $: 1$)