Experiment 14: Determining Planck's Constant

Purpose

The purpose of this experiment is to calculate the value of Planck's constant using the spectrum of different colored LED.

Experiment

The circuit shown above is built using 360ohms resistor and LED

Experiment Setup

The colors of LED used in this experiment: red, yellow, green and blue.

Spectra of red LED

Spectra of yellow LED

Spectra of green LED

Spectra of blue LED

Spectra of white LED

The spectrum of white LED consists of rainbow color, however, each color is clearly defined as if there's a separation gap between each color.

Data and Analysis

    Data from measurement:

    
Calculations from the measured data

Calculated Planck's constant:

LED Color	Wavelength(nm)	Voltage(V)	Calculated h (Js)	Error(%)
Red	607	1.890	6.12E-34	7.74
Green	514	2.579	7.07E-34	6.58
Yellow	570	1.920	5.75E-34	13.24
Blue	456	2.654	6.46E-34	2.63
Average			6.35E-34	4.26

From the data, we can see that the LED with longer wavelength has lower  measured voltage.

The sky seem blue because blue light has higher frequency, making it easier to scatter. The sunset looks red because the red light has lower frequency, thus it concentrates in the direction of the sun.

Calculated planck's constant from the graph

LED color	Wavelength(nm)	1/V (1/V)
Red	607	0.529
Green	514	0.388
Yellow	570	0.521
Blue	456	0.377

The equation of the graph: y = 7.46E-7x+ 1.98E-7
Slope = 7.46E-7x = hc/e

h = 3.98E-34 J*s

% Error = 40.0%

Conclusion

The measured Planck's constant using graphical method has much higher percent error compared to when each point is calculated individually. This shows that in order to make the graphical method to be more accurate, we need more data points.