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).

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).

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

In a Fresnel's biprism experiment,a mica sheet of refractive index $1.5$ and thickness $6 \times 10^{-6} \ m$ is placed in the path of one of the interfering beams. As a result,the central fringe shifts by a distance equal to $5$ fringe widths. Find the wavelength of light in $\mathring{A}$.

An oil film flowing on water appears coloured due to interference. For observing this effect,the approximate thickness of the oil film should be:

Which of the following phenomena produces the colour in a soap bubble?