Two spherical black bodies have radii ' $R_1$ ' and ' $\mathrm{R}_2$ '. Their surface temperatures are ' $\mathrm{T}_1$ ' and ' $T_2$ '. If they radiate same power, then $\frac{R_2}{R_1}$ is

Lorentz magnetic force is acting on a particle of charge q moving with velocity $\vec{V}$ in a magnetic field $\vec{B}$. The work done by this force on the charged particle is

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