Interference filter (mercury lines)
Infrared 1014 nm Red 623 nm Yellow 578 nm Green 546 nm Blue 436 nm Violet 405 nm
Iris 60%
Voltage measurement
Impedance Ordinary
Model of light
Photons (E = hν) Classical wave
Discharge Reset
▶▶ Automatic run over the six filters
Capacitor voltage : 0.00 V
Photoelectric current : 0.0 nA · time 0.0 s
Final voltage measured V0 : —
Electron energy E = e·V0 : —
Recorded points : 0
Slope of the line : —
Work function W: —
Threshold frequency ν0 : —
Choose a filter and press ILLUMINATE : the photons tear electrons out of the cathode, they reach the ring and charge the capacitor . As the voltage rises the electrons are pushed back, until they all stop: the final voltage V0 measures their energy. Change the iris : the current and the charging time change, V0 does not . Change the colour : V0 changes. The four filters marked ● are those measured in the thesis. The least-squares line gives the slope h/e , the work function and the threshold frequency . With the ordinary voltmeter the current flows back through the instrument and the capacitor never reaches V0 : an impedance buffer is needed. With the classical wave model the energy would depend on the intensity and not on the frequency: that is not what is observed.