Planck's Constant from an LED

Purpose:
The purpose of this experiment is to measure the Planck's constant using different LEDs. The experiment is conducted based on the concept of  the light emitting diode and interfere constructively of light going through a diffraction grating.
Procedure:

LEDs are connected to the DC power supply and the resistance box. Adjusting a resistance box to make the LEDs on with minimum voltage across the LEDs. The voltages across are recorded.

Shining the LEDs through the diffraction grating. Another person is looking through the grating to see the interference. The distance of the grating to the LEDs and the distance of interference are recorded.

Calculation of wavelength:

Derivation of Planck's constant:

Data table:

LED	Voltage (V)	Wavelength (nm)	h (Js) *10-34	% Error (%)
red	1.56	670 ± 18	5.57 ± 1	15.9
green	1.87	551 ± 15	5.49 ± 1	17.1
blue	2.25	485 ± 14	5.82 ± 1	12.1
yellow	1.63	594 ± 14	5.20 ± 1	21.5

The slope is the value of Voltage times Wavelength.
From the slope, the Planck's constant is equal to (slope*q)/c = 5.33E-34
% error is 19.5%

Summary:
According to the purpose of this experiment, the Planck constant  was successfully measured by using the electromagnetic radiation and light emitting diode. However, the result was not accurate due on the systematic measurement of the voltage across the LEDs and the wavelengths. The wavelengths were calculated based on the spectrum of LED light through the diffraction grating. As doing the experiment, we observed that not every LED give the single color when we looked through the grating. The green LED gave a mixture of colors in the spectrum. Moreover, the relationship between the color of the LEDs (wavelength) and its potential (voltage) is inversely proportional.