Dark and colour patterns on thin soap films are due to

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

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,by using light of wavelength $5000 \ \mathring{A}$,$5 \,mm$ wide fringes are obtained on a screen $1.0 \,m$ away from the coherent sources. The separation between the two coherent sources is.....$mm$.

$A$ thin slice is cut out of a glass cylinder along a plane parallel to its axis. The slice is placed on a flat glass plate as shown. The observed interference fringes from this combination shall be

When petrol leaking from a vehicle falls on a rain-wet road,colours are observed. This is due to the.......... of light.