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

$A$ photocell is receiving light from a source placed at a distance of $1 \ m$. If the same source is placed at a distance of $2 \ m$,then the ejected electrons:

$A$ photocell stops emission if it is maintained at $2V$ negative potential. The energy of the most energetic photoelectron is:

The figure shows the variation of photocurrent with anode potential for four different radiations. Let $I_a, I_b, I_c$ and $I_d$ be the intensities for the curves $a, b, c$ and $d$ respectively $[f_a, f_b, f_c$ and $f_d$ are frequencies respectively].

The number of photoelectrons emitted for light of frequency $v$ (higher than the threshold frequency $v_0$) is proportional to

What is the photoelectric effect?