What is the energy of a photon whose wavelength is $6840\,\mathring{A}$? (in $eV$)

The photoelectric effect can be understood on the basis of

When a photon enters glass from air,which one of the following quantities does not change?

In the Compton scattering process,the incident $X$-radiation is scattered at an angle of $60^{\circ}$. The wavelength of the scattered radiation is $0.22 \ \text{Å}$. The wavelength of the incident $X$-radiation in $\text{Å}$ units is:

Which scientist discarded the postulate of ether?

In pair annihilation,an electron and a positron destroy each other to produce gamma radiation. How is the momentum conserved?