A galvanometer of resistance $50 \Omega$ is converted into a voltmeter of range $(0-2 \mathrm{~V})$ using a resistor of $1.0 \mathrm{k} \Omega$. If it is to be converted into a voltmeter of range ( $0-10 \mathrm{~V}$ ), the resistance required will be

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