A coil has inductance 2 H . The ratio of its reactance when it is connected first to an a.c. source and then to a d.c. source is

In n-type semiconductor, free electrons donated by the impurity atoms occupy energy levels in

Two bodies A and B at temperatures ' $\mathrm{T}_1$ ' K and ' $\mathrm{T}_2$ ' K respectively have the same dimensions. Their emissivities are in the ratio $16: 1$. At $\mathrm{T}_1=\mathrm{xT}_2$, they radiate the same amount of heat per unit area per unit time. The value of $x$ is

Two polaroids are placed in the path of unpolarised beam of intensity ' $\mathrm{I}_0$ ' such that no light is emitted from the second polaroid. If a third polaroid whose polarisation axis makes an angle ' $\theta$ ' with the polarisation axis of first polaroid is placed between these polaroids then the intensity of light emerging from the last polaroid will be