The condition under which a microwave oven heats up a food item containing water molecules most efficiently is

A plano convex lens fits exactly into a plano concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive indices $$\mu $$₁ and $$\mu $$₂ and R is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is

Two pith balls carrying equal charges are suspended from a common point by strings of equal length, the equilibrium separation between them is r. Now the strings are rigidly clamped at half the height. The equilibrium separation between the balls now become

In Young's double slit experiment, the slits are 2 mm apart and are illuminated by photons of two wavelengths $${\lambda _1}$$ = 12000 $$\mathop A\limits^ \circ $$ and $${\lambda _2}$$ = 10000 $$\mathop A\limits^ \circ $$. At what minimum distance from the common central bright fringe on the screen 2 m from the slit will a bright fringe from one interference pattern coincide with a bright fringe from the other ?