In a biprism experiment, the $4^{th}$ dark band is formed opposite to one of the slits. The wavelength of light used is (where $D$ = distance between source and screen, $d$ = distance between the slits).

If Fresnel's biprism experiment is performed in water instead of air,what will be the effect on the fringe width?

In a biprism experiment,$21$ fringes are observed in a given region using light of wavelength $4800 \text{ Å}$. If light of wavelength $5600 \text{ Å}$ is used,the number of fringes in the same region will be:

To ensure almost $100$ per cent transmittivity,photographic lenses are often coated with a thin layer of dielectric material. The refractive index of this material is intermediate between that of air and glass (which makes the optical element of the lens). $A$ typically used dielectric film is $MgF_2$ $(n = 1.38)$. What should the thickness of the film be so that at the center of the visible spectrum $5500 \, \mathring{A}$ there is maximum transmission?

In a Fresnel biprism experiment,the two positions of the lens give separations between the virtual slits as $16 \; cm$ and $9 \; cm$ respectively. What is the actual distance of separation (in $cm$)?

When exposed to sunlight,thin films of oil on water often exhibit brilliant colours due to the phenomenon of