(a) Two energy levels of an electron in hydrogen atom are separated by 2.55 eV . Find the wavelength of radiation emitted when the electron makes transition from the higher energy level to the lower energy level.

(b) In which series of hydrogen spectrum this line shall fall?

The earth revolves around the sun in an orbit of radius $1.5 \times 10^{11} \mathrm{~m}$ with orbital speed $30 \mathrm{~km} / \mathrm{s}$. Find the quantum number that characterises its revolution using Bohr's model in this case (mass of earth $\left.=6.0 \times 10^{24} \mathrm{~kg}\right)$

(a) Write Einstein's photoelectric equation. How did Millikan prove the validity of this equation?

(b) Explain the existence of threshold frequency of incident radiation for photoelectric emission from a given surface.

(a) Define the term 'electric flux' and write its dimensions.

(b) A plane surface, in shape of a square of side 1 cm is placed in an electric field $\vec{E}=\left(100 \frac{\mathrm{~N}}{\mathrm{C}}\right) \hat{i}$ such that the unit vector normal to the surface is given by $\hat{n}=0.8 \hat{i}+0.6 \hat{k}$. Find the electric flux through the surface.