Trying to find the right answer to this..

Wave Optics

Question

Quarks inside protons and neutrons are thought to carry fractional charges [(+ 2/3)e; (–1/3e). Why do they not show up in Millikan’s oil-drop experiment?

Answer

Quarks are thought to be confined within a proton or neutron by forces which grow stronger if one tries to pull them apart. That is the reason, the quarks always remain together. The fact is, though fractional charges exist in nature, the observable charges are always integral multiple of charge of electron.