Determining Planck's Constant from an LED Lab

The purpose of this experiment is to successfully approximate Planck's constant while using different Light Emitting Diodes (LED).

Planck's constant is found in the expression which involves energy and frequency

E = hf = hc/λ

where the theoretical value of h is 6.626*10^-34 Js

In order to measure the electric field E, we can relate the concept of electric potential difference, or voltage;

E = qV, where q is the charge of an electron = 1.6*10^-19 C

Equating both Es, we can get the following formula

hc/λ = qV

and solving for h yields;    h = λqV/c

Voltage delivered by a power supply will be measured just when the LEDs are dimly  lit. This will be achieved by controlling the resistance of the circuit, avoiding the resistance to be zero since too much voltage would toast the LEDs.

λ is obtained using the method of spectrum lines through the formula:

λ = Dd/(L^2 +D^2 )^1/2

We then recorded the following data:

% error = |exp - act|/act *100 for Planck's constant in each of the LED lights:

Actual Value: 6.626*10^-34 Js

Green LED: 17.30%
Blue  LED: 22.88%
Yellow LED: 25.45%
Red LED: 21.52%

The percent error was moderately low giving the green LED the lowest percent error, where as the yellow LED threw the largest percent error out of the 4 samples.

One thing worth mentioning is that even though LEDs have a particular color, when they were seen through the diffraction grating, there was still a spectrum of different colors. This happened with all the 4 LEDs.

Also, when reading voltages, we saw that the blue LED particularly, changed to green if we varied the resistance. This means that wavelength is related to the voltage delivered to the LED.