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?

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

What is the frequency of violet light having wavelength $400 \ nm$?

$A$ hypothetical electromagnetic wave is shown in the figure. Find out the wavelength of the radiation.

Identify the correct statements from the following:
$I$) Isotopes of an element show different chemical behaviour
$II$) Lyman series of lines of hydrogen spectrum appear in $UV$ region
$III$) The oscillating electric and magnetic field components of electromagnetic radiation are perpendicular to each other and both are perpendicular to the direction of propagation of radiation

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