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

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 $....$.

The frequency of a radio wave from Mars to Earth is $6 \times 10^3 \, s^{-1}$. If the distance between them is $8 \times 10^7 \, km$,how much time will it take?

What is an emission spectrum?

The photoelectric effect can be caused by which of the following?

Explain the dual behavior of electromagnetic radiation.