The figure shows how the photocurrent of a photocell depends on the potential of its collector, for three narrow beams of single-colour light, 1, 2 and 3. The beams fall in turn on the same cathode in the same tube. The stopping potentials and the saturation currents are read straight from the curves. Which conclusion can be drawn from the figure alone?

Fig. 1: A graph of photocurrent in microamperes, on the vertical axis from 0 to 5, against the potential of the collector in volts, from minus 4 to plus 4. Three curves, 1, 2 and 3, for three beams on the same cathode, rise from zero and level off. Curves 1 and 2 leave zero at minus 1.5 V; curve 1 levels off at 2.0 microamperes and curve 2 at 4.0 microamperes. Curve 3 leaves zero at minus 3.0 V and levels off at 4.0 microamperes.