A person can see objects clearly when they lie between 40 cm and 400 cm from his eye. In order to increase the maximum distance of distant vision to infinity the type of lens and power of correction lens required respectively, are

If a slit of width $x$ was illuminated by red light having wavelength $6500\mathop {\rm{A}}\limits^{\rm{^\circ }}$, the first minima was obtained at $\theta=30^{\circ}$. Then, the value of $x$ is

A neutral ammonia $\mathrm{NH}_3$ in its vapour state has electric dipole moment of magnitude $5 \times 10^{-30} \mathrm{C}-\mathrm{m}$. How far apart are the molecules centres of positive and negative charge?

If four charges $q_1=+1 \times 10^{-8} \mathrm{C}, q_2=-2 \times 10^{-8} \mathrm{C}$, $q_3=+3 \times 10^{-8} \mathrm{C}$ and $q_4=+2 \times 10^{-8} \mathrm{C}$ are kept at the four corners of a square of side 1 m , then the electric potential at the centre of the square is