$A$ bulb of $40 \ W$ is producing light of wavelength $620 \ nm$ with $80 \%$ efficiency. The number of photons emitted by the bulb in $20 \ s$ is $(1 \ eV = 1.6 \times 10^{-19} \ J, hc = 12400 \ eV \ \mathring{A})$

$A$ $200 \ W$ bulb emits monochromatic light of wavelength $200 \ nm$. Calculate the number of photons emitted per second by the bulb.

The photoelectric behaviour of $K$,$Li$,$Mg$,and $Ag$ metals is shown in the plot below. If light of wavelength $400 \ nm$ is incident on each of these metals,which of them will emit photoelectrons?
[Planck's constant $h=6.626 \times 10^{-34} \ J \ s$; velocity of light $c=3 \times 10^8 \ m \ s^{-1}$; $1 \ eV = 1.6 \times 10^{-19} \ J$]

The frequency of radiation emitted when the electron falls from $n = 4$ to $n = 1$ in a hydrogen atom will be (Given ionization energy of $H = 2.18 \times 10^{-18} \ J \ atom^{-1}$ and $h = 6.625 \times 10^{-34} \ Js$)

The frequency and energy of a photon with a wavelength of $4000 \ \mathring{A}$ are respectively.......

The wavelength of a spectral line emitted by a hydrogen atom in the Lyman series is $\frac{16}{15 R} \ cm$. What is the value of $n_2$? $(R = \text{Rydberg constant})$