Quantam nature of light is explained by which of the following phenomenon

The photo-electrons emitted from a surface of sodium metal are such that

The work functions of Caesium $(Cs)$,Potassium $(K)$ and Sodium $(Na)$ are $2.14\,eV,2.30\,eV$ and $2.75\,eV$ respectively. If incident electromagnetic radiation has an incident energy of $2.20\, eV$, which of these photosensitive surfaces may emit photoelectrons?

A beam of light of wavelength $400\,nm$ and power $1.55\,mW$ is directed at the cathode of a photoelectric cell. If only $10 \%$ of the incident photons effectively produce photoelectron, then find current due to these electrons $...........\mu A$

[given, $hc =1240\,eV - nm , e =1.6 \times 10^{-{ }^{19}\,C }$ )

A caesium photocell, with a steady potential difference of $60V$ across, is illuminated by a bright point source of light $50 cm$ away. When the same light is placed 1m away the photoelectrons emitted from the cell

A $160 \,W$ infrared source is radiating light of wavelength $50000 \,\mathring A$ uniformly in all directions. The photon flux at a distance of $1.8 \,m$ is the order of ............. $m^{-2} s ^{-1}$