In photoelectric effect what is meant by the terms: stopping potential?

Every metal has a definite work function. Even then, when light of a single frequency (monochromatic) is incident on a metal, photoelectrons having different energies are emitted, why?

When monochromatic radiation of wavelength $2000 \stackrel{\mathrm{o}}{A}$ falls upon a nickel plate, the later acquires a positive charge. When the wavelength is increased, at $3400 \stackrel{\mathrm{o}}{A}$ the effect is found to cease, however intense the incident radiation may be. Explain it.

The work functions of $\mathrm{Na}, \mathrm{K}, \mathrm{Mo}$ and $Ni$ are $1.92 \mathrm{eV}, 2.15 \mathrm{eV} 4.17 \mathrm{eV}$ and $5.0 eV$ respectively. Which of these metals will not give photoelectric emission for radiation of wavelength $3300 \stackrel{\mathrm{o}}{A}$ from a $\mathrm{He}-\mathrm{Cd}$ laser source placed $1.0 \mathrm{m}$ away from the photocell? What would happen if the laser be brought nearer and placed $0.5 \mathrm{m}$ away?

The figure shows a plot of cut-off voltage versus frequency of incident radiation for photoelectric effect in sodium. Calculate the threshold frequency and the work function of sodium. $\left(h=6.6\times {10}^{-34} \mathrm{Js}\right)$

The figure shows graphs between the stopping potential $V_0$ and the frequency $v$ of the incident radiation on two different metal plates $P$ and $Q$. Which metal, $P$ or $Q$, has the greater value of work function?

The figure shows graphs between the stopping potential $V_0$ and the frequency $v$ of the incident radiation on two different metal plates $P$ and $Q$. What does the slope of the lines depict?