In the hydrogen spectrum,if an electron transitions from the $6^{th}$ orbit to the $3^{rd}$ orbit,what is the total number of spectral lines produced?

What transition in the hydrogen spectrum has the same wavelength as the Balmer transition,$n=4$ to $n=2$,of the $He^{+}$ spectrum?

Which one of the following about an electron occupying the $1s$ orbital in a hydrogen atom is incorrect? (The Bohr radius is represented by $a_0$)

Angular momentum of an electron in the $M$-shell of a $Li^{+2}$ ion is:

The wavelength of the spectral line obtained in the spectrum of $Li^{2+}$ ion, when the transition takes place between two levels whose sum is $4$ and difference is $2$, is

$A$ $100 \, W$ bulb emits monochromatic light of wavelength $400 \, nm$. Calculate the number of photons emitted per second by the bulb.