Initially interference is observed with the entire experimental set-up inside a chamber filled with air. Now the chamber is evacuated. With the same source of light used, a careful observer will find that

Two coherent light sources each of wavelength $\lambda$ are separated by a distance $3\mathrm{\lambda }$. The total number of minima formed on the line $AB$ which runs from $–\infty \text{to} +\infty$ is,

In the figure shown, if a parallel beam of white light is incident on the plane of the slits, then the distance of the nearest white spot on the screen from $O$ is $[\text{assume,} d<<D\text{,} \lambda <<d]$, 

The Young's double slit experiment is performed with blue light and green light of wavelengths $4360 \mathrm{\AA }$ and $5460 \mathrm{\AA }$ respectively. If $X$ is the distance of $4^{th}$ maxima from the central one, then