The number of photoelectrons emitted varies with which physical quantity?

Explain Hertz's observation in short.

In the photoelectric effect, keeping the frequency of incident radiation and the accelerating potential fixed, if the intensity of incident light is increased,

Threshold frequency of which of the following metals does not lie in the ultraviolet region (in the case of the photoelectric effect)?

Do all the electrons that absorb a photon come out as photoelectrons?

$A$ photocell is illuminated by a small bright source placed $d \; m$ away. When the same source of light is placed $\frac{d}{2} \; m$ away,the number of electrons emitted by the photocathode would