What is the intensity of radiation (light)?

Which graph represents the relationship between the intensity of incident light on a metal plate and the resulting photoelectric current?

In a photoelectric experiment,keeping the frequency of incident radiation and the accelerating potential fixed,if the intensity of incident light is increased,then:

$A$ photoelectric cell is illuminated by a point source of light placed at a distance of $1 \ m$. If the source is moved to a distance of $2 \ m$,then:

Describe Hallwach's and Lenard's observations regarding the photoelectric effect in short.

The figure shows the variation of photocurrent $i$ with anode potential $V$ for three different radiations. Let $I_a, I_b$ and $I_c$ be the intensities and $f_a, f_b$ and $f_c$ be the frequencies for the curves $a, b$ and $c$ respectively. Then