Two coils are placed near each other. When the current in one coil is changed at the rate of $5 \mathrm{~A} / \mathrm{s}$, an emf of 2 mV is induced in the other. The mutual inductance of the two coils is

The electromagnetic waves used to purify water are

The focal lengths of the objective and the eyepiece of a compound microscope are 1 cm and 2 cm respectively. If the tube length of the microscope is 10 cm , the magnification obtained by the microscope for most suitable viewing by relaxed eye is

The variation of the stopping potential $\left(\mathrm{V}_0\right)$ with the frequency $(v)$ of the incident radiation for four metals A, B, C and D is shown in the figure. For the same frequency of incident radiation producing photo-electrons in all metals, the kinetic energy of photo-electrons will be maximum for metal