Graph shows the variation of fringe width ( X ) versus distance of the screen from the plane of the slits (D) in Young's double slit experiment. (keeping other parameters same, $d=$ distance between the slits). The wavelength of light used can be calculated as

The point charges $+\mathrm{q},-\mathrm{q},-\mathrm{q},+\mathrm{q},+\mathrm{Q}$ and -q are placed at the vertices of a regular hexagon ABCDEF as shown in figure. The electric field at the centre of hexagon ' $O$ ' due to the five charges at $\mathrm{A}, \mathrm{B}, \mathrm{C}, \mathrm{D}$ and F is twice the electric field at centre ' O ' due to charge +Q at E alone. The value of $Q$ is

$$ \text { The frequency at resonance for the circuit is } $$

The work done in splitting a water drop of radius R into 64 droplets is ( $\mathrm{T}=$ surface tension of water)