Plank's Constant Lab

In this experiment we were trying to measure the planks constant. The setup for the experiment is the same as light spectra setup with a few differences. First of all, the light source instead of a heated gas are 4 LEDs, blue, green, yellow and red.  Also for this experiment we used a voltage source to illuminate the LED's since those LEDs did not work with AC power. From the experiment we obtained voltage across the LEDs and also the associated wavelengths for the LEDs.

d= 2*10^-6 m = 500 slits per mm

D= distance of the light spectra 

L= 0.97 ± 0.005 m

The picture above shows both the green LED and its view through the diffraction grating.

as you can see it is really hard to point at a position with high precision for D. That is the reason we end up getting a huge uncertainty to compensate for the measurement difficulties.

Next is the table for λ and its associated voltage and the graph of voltage vs 1/λ.

                        ±0.001              ±42767

The horizontal axis on the graph is the 1/λ and the vertical is the voltage.

Using the following formula we calculated the planks constant:

where e is charge of an electron and V is the voltage measured across the LED by a meter and finally c is the speed of light.

e = 1.602E-19

c=3E8

The values calculated from measurement all agree with h within uncertainty. However the most amount of error is associated with green and yellow which as mentioned before might be due to the fact that while we were looking through the grating the least resolved and sharp light lines were from green and yellow and that is why they are both from the actual value by 7 and 10 percent respectively.