What is the effect on Fresnel's biprism experiment when white light is used?

$A$ thin film of soap solution $(n = 1.4)$ lies on the top of a glass plate $(n = 1.5)$. When visible light is incident almost normal to the plate,two adjacent reflection maxima are observed at two wavelengths $420 \ nm$ and $630 \ nm$. The minimum thickness of the soap solution is.....$nm$

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

Light of wavelength $6000 \, \mathring{A}$ is incident on a thin glass plate of refractive index $1.5$ such that the angle of refraction in the plate is $60^\circ$. Find the thickness of the plate so that it appears dark in reflection.

In a biprism experiment,monochromatic light of wavelength $\lambda$ is used. The distance between the two coherent sources $d$ is kept constant. If the distance between the slit and the eyepiece $D$ is varied as $D_1, D_2, D_3, D_4$ and the corresponding measured fringe widths are $W_1, W_2, W_3, W_4$,then:

In a biprism experiment, a steady interference pattern is observed on the screen kept at a distance of $100 \text{ cm}$ using a light of wavelength $5000 \text{ Å}$. Without changing the distance between the virtual sources of the slit, the source of light is replaced by a source of wavelength $6400 \text{ Å}$. Now, to reduce the fringe width by $20\%$ of its initial value, the screen should be moved