Which of the following does not characterise $X$-rays?

Electrons are emitted with zero velocity from a metal surface when it is exposed to radiation of wavelength $8000 \ \mathring{A}$. What should be the maximum velocity of emitted electron when the same metal surface is exposed to radiation of wavelength $5000 \ \mathring{A}$? $\left( h = 6.6 \times 10^{-34} \ J \cdot s, \ m_e = 9.1 \times 10^{-31} \ kg, \ c = 3 \times 10^8 \ m/s \right)$

Explain the reason for the formation of a spectrum.

The frequency of radiation with a wavelength of $5800 \ \mathring{A}$ is ... $s^{-1}$.

What is the wavelength for a wave having frequency $50 \ Hz$?

The wavelength (in $\mathring{A}$) of a photon having energy $3 \ eV$ is approximately.
$1 \ eV = 1.6 \times 10^{-12} \ erg$
$h = 6.626 \times 10^{-27} \ erg \ s$
$c = 3 \times 10^{10} \ cm/s$