What is light?

Fill in the blank: Wave number is measured in units of .........

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 $....$.

How is an electron observed? Explain.

The wavelength of a spectral line of caesium is $460 \ nm$. What is the frequency of the spectral line?

What is an emission spectrum?