In a photoelectric effect set-up, a point source of light of power $3.2\times {10}^{-3}W$ emits monochromatic photon of energy $5eV$. The source is located at a distance o $0.8m$ from the centre of a stationary metallic sphere of work function $3eV$ and of radius $8\times {10}^{-3}m$. The efficiency of photoelectron emission is one for every  ${10}^6$ incident photons. Assume that the sphere is isolated and electrons are instantly swept away after emission.

It is observed that the photoelectron emission stops at a certain time $t$ after the light source switched on. Why?