In a photoelectric experiment in which monochromatic light, and a sodium photocathode are used, we nd a stopping potential of 1.85 V for 7L=3000 A, and of 0.82 V for l=4000 A. From these relations derive: (a) the value of Planck's constant; (b) the workfunction of sodium in electron volts; (c) the threshold wavelength for sodium