Walk the Planck

Wenjun Li and Douglas Richmond

Department of Physics, University of Wisconsin-Parkside 900 Wood Road, Kenosha, Wisconsin 53141 USA (Dated: March 22, 2006)

Experimentally verify Planck’s Hypothesis and recover Planck’s constant.

I. INTRODUCTION By graphing the minimum voltage V _min of the electron emission of six different LEDs versus inverse of their wavelength, we are able to determine the Planck’s constant h from the slope of the graph.
II. PROCEDURE A. Instruments and materials 1. LED Apparatus
2. Variable AC power supply
3. (2) Multi-voltmeters
B. The Experiment For each of the LED elements we ran an accelerating voltage through it’s relative circuit, and tested the current at a pre-decided threshold of 20.0 Volts. We then lowered the current until the LED turned off. Since we were looking for about 20 data points, we did a quick approximate division, and hence, had our average step voltage between data points. Repeating this for all of the LED elements we recovered the voltage, and the amperage at each step, and recorded them for data analysis.
III. ANALYSIS AND CONCLUSIONS To analyze the data we obtained, we graphed it as Voltage (V) vs. Current (I), ddVI- vs. Current, and ln( I-
I0 + 1) vs. Voltage (V ). These are respectively represented below for each wavelength of LED that we garnered data from. 480 nm

560 nm

590 nm

635 nm

665 nm

950 nm

IV. PLANCK’S CONSTANT To determine Planck’s constant we graphed the minimum Voltage vs. -1
λ of each diode. We performed a linear regression and found the slope of the resulting line to be 1410.351