For the plane electromagnetic wave given by $$E=E_{0} \sin (\omega t-k x)$$ and $$B=B_{0} \sin (\omega t-k x)$$, the ratio of average electric energy density to average magnetic energy density is

The resistivity $$(\rho)$$ of semiconductor varies with temperature. Which of the following curve represents the correct behaviour :

The energy levels of an hydrogen atom are shown below. The transition corresponding to emission of shortest wavelength is :

The number of air molecules per cm$$^3$$ increased from $$3\times10^{19}$$ to $$12\times10^{19}$$. The ratio of collision frequency of air molecules before and after the increase in number respectively is: