Four electric charges $$+\mathrm{q},+\mathrm{q},-\mathrm{q}$$ and $$-\mathrm{q}$$ are placed in order at the corners of a square of side $$2 \mathrm{~L}$$. The electric potential at point midway between the two positive charges is

White light consists of wavelengths from $$480 \mathrm{~nm}$$ to $$672 \mathrm{~nm}$$. What will be the wavelength range when white light is passed through glass of refractive index 1.6?

A metal sphere cools at the rate of $$1.5^{\circ} \mathrm{C} / \mathrm{min}$$ when its temperature is $$80^{\circ} \mathrm{C}$$. At what rate will it cool when its temperature falls to $$50^{\circ} \mathrm{C}$$. [Temperature of surrounding is $$30^{\circ} \mathrm{C}$$]

The ratio of energies of photons produced due to transition of electron of hydrogen atom from its (i) second to first energy level and (ii) highest energy level to second energy level is