What is interference in the context of electromagnetic radiation?

In the hydrogen spectrum,the various lines of the Lyman series lie in the $.......$ region.

The momentum (in $kg \cdot m/s$) of a photon having $6 \, MeV$ energy is:

Calculate the wavenumber $\bar{v}$ in $cm^{-1}$ for yellow radiation having a wavelength of $5800 \ \mathring{A}$.

Which of the following statements is correct about the photoelectric effect?
$1$. The number of electrons ejected from a metal surface is inversely proportional to the intensity of light.
$2$. Below the threshold frequency,the photoelectric effect can be observed.
$3$. At a frequency higher than the threshold frequency,the ejected electrons have a certain kinetic energy.
$4$. At a frequency higher than the threshold frequency,the electron is still on the metal surface.

The number of the following statements which is/are incorrect is:
$A$. Line emission spectra are used to study the electronic structure.
$B$. The emission spectra of atoms in the gas phase show a continuous spread of wavelength from red to violet.
$C$. An absorption spectrum is like the photographic negative of an emission spectrum.
$D$. The element helium was discovered in the sun by spectroscopic method.