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

Fill in the blanks:
$(i)$ If the orbits around the nucleus contract,the electron will spiral into the nucleus in $....$ seconds.
$(ii)$ Waves used in radio broadcasting have a frequency of $....$ $Hz$.
$(iii)$ The $SI$ unit of frequency is $....$.
$(iv)$ The number of waves present in unit length is called $....$.

Calculate the wavelength,frequency and wave number of a light wave whose period is $2.0 \times 10^{-10} \,s$.

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?

$A$ 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 : ................. $nm$

Calculate $(a)$ wavenumber and $(b)$ frequency of yellow radiation having wavelength $5800 \, \mathring{A}$.