A circular hole of radius $$\left( {{a \over 2}} \right)$$ is cut out of a circular disc of radius 'a' as shown in figure. The centroid of the remaining circular portion with respect to point 'O' will be :

Zener breakdown occurs in a p $$-$$ n junction having p and n both :

The de-Broglie wavelength of a proton and $$\alpha$$-particle are equal. The ratio of their velocities is :

According to Bohr atomic model, in which of the following transitions will the frequency be maximum?