The half life of a radioactive substance is 20 minutes. In how much time, the activity of substance drops to $$\left(\frac{1}{16}\right)^{\text {th }}$$ of its initial value?

In hydrogen spectrum, the shortest wavelength in the Balmer series is $$\lambda$$. The shortest wavelength in the Bracket series is :

The radius of inner most orbit of hydrogen atom is $$5.3 \times 10^{-11} \mathrm{~m}$$. What is the radius of third allowed orbit of hydrogen atom?

Let R₁ be the radius of the second stationary orbit and R₂ be the radius of the fourth stationary orbit of an electron in Bohr's model. The ratio $${{{R_1}} \over {{R_2}}}$$ is :