Experiment 17: Planck's Constant

Purpose: To experimentally determine Planck's constant using the diffraction method and LED lights.

Set Up: Just as in the hydrogen spectra lab, we measured the light spectra through a diffraction grating for a red, yellow, blue, and green LED.

Yellow:

Blue:

                                   

Green:

Red:

White:

Then we calculated the wavelength associated with each color and recorded the voltage across each LED.

Data:

d	2.00E-06
	Yellow	Red	Blue	Green
Length (m)	1.80	1.80	1.80	1.80
Voltage (V)	1.914	1.907	2.660	2.830
Gap 1	0.5233	0.5587	0.4727	0.5133
Gap 2	0.5453	0.5713	0.4706	0.5152
Gap 3	0.5333	0.5883	0.4705	0.5030
Average	0.5339	0.5727	0.4713	0.5105
Lambda	5.687E-07	6.064E-07	5.065E-07	5.457E-07
eV	3.062E-19	3.051E-19	4.256E-19	4.528E-19
C/eV	9.796E+26	9.832E+26	7.049E+26	6.625E+26
h	5.806E-34	6.168E-34	7.186E-34	8.236E-34
average h	6.849E-34

We can rewrite E = hf    as     lamba = hc/E and the slope is the value for h.

Conclusion: The value we obtained for h is 2E-34 which has percent error of 70.43% to the actual value measured to be 6.626E-34. The error can be contributed to the strange voltage we measured for the blue LED.