White light reflected from a soap film (Refractive Index $\mu = 1.5$) has a maxima at $600 \ nm$ and a minima at $450 \ nm$ with no minimum in between. Then the thickness of the film in units of $10^{-7} \ m$ is:

When white light is incident normally on an oil film of thickness $10^{-4} \,cm$ and refractive index $1.4$,the wavelength which will not be seen in the reflected system of light is ........... $\mathring{A}$.

The following diagram is an arrangement to view the interference pattern produced by a wedge of liquid between a microscope slide and a glass block. The interference pattern obtained is made of equally spaced parallel fringes. The fringe separation (or fringe width) $\beta$ may be decreased by some of the following actions (Consider one change at a time).

It is necessary to coat a glass lens with a non-reflecting layer. If the wavelength of the light in the coating is $\lambda$,the best choice is a layer of material having an index of refraction between those of glass and air and a thickness of

In a Fresnel's biprism experiment,fringes are obtained in the focal plane of an eyepiece at a distance of $1 \ m$ from the slit. $A$ convex lens is placed between the biprism and the eyepiece. The images of the slit are formed at two positions of the lens. The distances between the two images of the slit at these two positions are $4.05 \times 10^{-3} \ m$ and $2.9 \times 10^{-3} \ m$ respectively. Calculate the distance between the slits.

Oil floating on water looks coloured due to interference of light. What should be the order of magnitude of the thickness of the oil layer in order that this effect may be observed? (in $m$)