Light emerges out of a convex lens when a source of light kept at its focus. The shape of wavefront of the light is :

An electron rotates in a circle around a nucleus having positive charge $$\mathrm{Ze}$$. Correct relation between total energy (E) of electron to its potential energy (U) is :

Time periods of oscillation of the same simple pendulum measured using four different measuring clocks were recorded as $$4.62 \mathrm{~s}, 4.632 \mathrm{~s}, 4.6 \mathrm{~s}$$ and $$4.64 \mathrm{~s}$$. The arithmetic mean of these readings in correct significant figure is :

The angle between vector $$\vec{Q}$$ and the resultant of $$(2 \vec{Q}+2 \vec{P})$$ and $$(2 \vec{Q}-2 \vec{P})$$ is :