For this post, I'll be mainly referring to this photoelectric equation: Fig 1: Photoelectric equation 1) Is 'e' the charge of an electron, ie. a negative value? Or just an elementary charge, ie. a positive value? 2) Is stopping potential (Vs) a positive or negative value? Because in my lecture notes, there's two different graphs, one saying that Vs is positive, and one saying that it's negative: Fig 2: Current against Potential difference graph Fig 3: Stopping potential vs Frequency graph 2) Googling 'stopping potential vs frequency graph' yields me almost similar results of this graph: Fig 4: Stopping potential vs Frequency graph Must threshold frequency necessarily be the x intercept? Can it be any higher value, like the one shown in Fig 3? Thanks!