Using LED to calucale plank's constant

Objective: The purpose of this experiment is to use the voltage measurements of different colors of LED light to find the plank’s constant. Same as laser, LED light is a highly monochromatic light too. This can be proved by adding a yellow coating over red LED light, and the light is gone. The color of LED light can be changed by increasing or decreasing the energy. According to equation λ=hc/E, with c and h holds constant, increasing and decreasing energy can turn the color of LED light from red to other light color of shorter wavelength, such as yellow, green, and blue.

In this experiment, the equation h=λ*E/c is used to find the plank’s constant. The actual wavelength of different color of LED light is used. The energy is obtained from the e*v. The experimental plank’s constant will be compared to the actual plank’s constant of 6.63*10^-34 m²kg/s.

Figure 1: The blue LED light is used. The total voltage used, the voltage of resistor, and the voltage of the LED are measured.

Figure 2: The yellow LED light is used.

Data

Color	Voltage (v)	Wavelength (nm)	Calculated h ( m2kg/s)	Percent Error (%)
Blue	2.76	590	8.69*10-34	31
Yellow	1.95	450	4.68*10-34	29

Sample Calculations:

h=λ*E/c

 = 590*10^-9*2.76*1.602*10^-19/ (3*10^8)

 =8.69*10^-34 m²kg/s

Using the data provide, a graph of voltage vs. 1/λ can be plotted.

The slope of the graph is Vλ, the plank’s constant can be calculated using slope=hc/e, with e=1.602*10^-19.

The slope is 2*10-6. Using using slope=hc/e, the slope is calculated to be 1.068*10^33

Conclusion

The percent error of the calculated plank’s constant compared to the actual one is large. To improve the experiment, more color of LED light should be used to obtain more experimental h values. Also, the wavelength used in the calculation should be the experimental wavelength instead of the actual one. To set up the experiment to measure the experimental wavelength, a color filter and a ruler are used. Using the equation y=Lλ/d, with L is the length from light source to slit, y is the distance between the color of interest to the light source and d is the distance between slits, the experimental wavelength can be measured.