$A$ very thin transparent film (like a soap bubble) is viewed in white light. The color of the film appears .......

In a biprism experiment,the fringe width is $0.6 \ mm$. The distance between the $6^{th}$ dark fringe and the $8^{th}$ bright fringe on the same side of the central bright fringe is: (in $mm$)

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.

In a Fresnel biprism experiment,which of the following light sources is used to identify the central fringe?

$A$ 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 $= \lambda$).

The approximate thickness of an oil film required to observe the interference of light (due to which it appears coloured) is .......