Planck's Constant Experiment







By measuring the voltage across the LED when it just switches on and knowing the wavelength of light emitted by the LED, we can calculate Planck’s constant (h). By assuming the energy lost by an electron (qVo) when it recombines with a hole is equal to the photon energy (hf = hc/wavelength), we can write:-
qV
 = hc/wavelength , or
h = qV
o wavelength/c

Where c is the speed of light, q = -1.6 x 10-19

C is the charge on the electron and Vo is the threshold voltage.



 



                                     Results of an experiment using the Planck's Constant apparatus.
  
LED Colour     LED Wavelength (nm)     LED Voltage      Wavelength (m)       Freq(Hz)                EnergyLost      

      Red                                644                       1.38                   6.44E-07             4.66E+14                 2.208E-19

    Yellow                             590                       1.52                   5.90E-07              5.08E+14                 2.432E-19              
   
Green                             535                       1.98                   5.35E-07              5.61E+14                  3.168E-19                                
      Blue                               440                       2.24                  4.40E-07              6.82E+14                 3.584E-19




















The slope of the line in the graph gives
6.542E-34
The actual value of Planck's constant is 
6.626E-34