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

In a biprism experiment,the distance between the two sources is doubled and the distance between the slit and the eyepiece is also doubled. Then the width of the fringe is

$A$ thin oil film of refractive index $1.2$ floats on the surface of water $(\mu = \frac{4}{3})$. When light of wavelength $\lambda = 9.6 \times 10^{-7} \ m$ falls normally on the film from air,it appears dark when seen normally. What is the minimum change in its thickness for which it will appear bright in normally reflected light by the same light?

$A$ plano-convex lens is placed with its curved surface on a plane glass plate,creating a thin air film. With monochromatic light,this thin film produces an interference pattern due to reflections from the top curved surface and the bottom glass plate surface.
Statement-$1$: When light is reflected from the air-glass plate interface,it undergoes a phase change of $\pi$.
Statement-$2$: The center of the interference pattern is dark.

The thickness of a plate having refractive index $\mu$ which will produce a change in optical path equal to half the wavelength $\lambda$ of the light passing through it is

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?