Calculate radius of third orbit of $$\mathrm{He}^{+}$$.

Which of the following electromagnetic radiations possesses lowest energy?

Calculate the wave number of photon emitted during transition from the orbit of $$n=3$$ to $$n=2$$ in hydrogen atom $$\left(R_H=109677 \mathrm{~cm}^{-1}\right)$$.

Find the radius of fourth orbit of hydrogen atom if its radius of first orbit is $$\mathrm{R}$$ pm.