The photoelectric effect is described as the ejection of electrons from the surface of a metal when :

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.

$A$ gas absorbs a photon of $355 \ nm$ and emits at two wavelengths. If one of the emissions is at $680 \ nm,$ the other is at : ................. $nm$

The work functions $(W_0)$ of $K, Na, Li, Mg$ and $Cu$ are $2.25, 2.30, 2.42, 3.70$ and $4.80 \ eV$ respectively. How many of these metals do not undergo photoelectric effect when a radiation of wavelength $450 \ nm$ is allowed to fall on them? $(1 \ eV = 1.602 \times 10^{-19} \ J)$

Work functions of four metals $M_1, M_2, M_3$ and $M_4$ are $4.8, 4.3, 4.75$ and $3.75 \ eV$ respectively. The metals which do not show photoelectric effect when light of wavelength $310 \ nm$ falls on the metals are

Find the wavelength of radiation having frequency $4 \times 10^{11} \ kHz$.