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

$A$ hypothetical electromagnetic wave is shown below. The frequency of the wave is $x \times 10^{19} \ Hz$. $x =$ . . . . (nearest integer)

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.

What is a black body?

The photoelectric effect demonstrates:

Which emission transition series is obtained when an electron jumps from $n_2 = \infty$ to $n_1 = 1$?