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 $cm$)?

$A$ ray of light of intensity $I$ is incident on a parallel glass slab at a point $A$ as shown in the figure. It undergoes partial reflection and refraction. At each reflection,$25\%$ of the incident energy is reflected. The rays $AB$ and $A^1B^1$ undergo interference. The ratio $I_{\text{max}} / I_{\text{min}}$ is (in $: 1$)

$A$ Fresnel's biprism experiment is performed in water. If the fringe width in air is $\beta_a$,and the refractive indices of the biprism material and water are $1.5$ and $1.33$ respectively,find the new fringe width.

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 $m$)

In Fresnel's biprism,coherent sources are obtained by ...

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