Determining Planck's Constant

In this experiment we were using LEDs (Light Emitting Diodes) to determine Planck's Constant experimentally. We tested two light emitting diodes which were blue and yellow. They shine at different levels of intensity at different voltages because of the color of light that they emit. The values for the wavelength(λ) of blue and yellow light were estimated. The picture below is of the an LED with a diffraction grating in front of the lens. The only color seen through the diffraction grating is a deep blue color and a pale yellow color.

The energy was determined by the voltage output coming from the power supply. When the electrons pass through a potential,  they will have an energy in terms of electron volts(eV).

E_Blue = 1.95 eV λ_Blue = 450 nm
E_Yellow = 2.76 eV λ_Yellow = 590 nm

The experimental value of Planck's constant can be determined by plotting the values of λ vs 1/E and using the slope. The slope can be used to calculate Planck's constant.

 

The slope from the graph is m = 1.490225*10^-25.  h can be found through this equation, 

m = hc, where m is the slope and it is a relation between the wavelength and the energy, h is Planck's constant, and c is the speed of light.  m/c = h 

h = 4.967 * 10^-34 Js is the calculated value. The percent error of this value compared to the actual value is  25%. We have this large error because only two colors were used to determine the slope and the values of the wavelength were estimated instead of calculated.