The laser in an audio compact disc uses light of wavelength $7.8 \times 10^2 \ nm$. What is the frequency of light emitted by the laser?

$X$-rays are produced due to

The work functions of $Ag, Mg, K$ and $Na$ respectively in $eV$ are $4.3, 3.7, 2.25, 2.30$. When an electromagnetic radiation of wavelength of $300 \ nm$ is allowed to fall on these metal surfaces,the number of metals from which the electrons are ejected is $(eV = 1.6022 \times 10^{-19} \ J)$

Mercury street lights emit light of a pale blue color with a wavelength of $436 \, nm$. Calculate its frequency.

Energy of a photon is $1 \ MeV,$ then its momentum will be $:-$

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? $(1 \ eV = 1.602 \times 10^{-19} \ J)$