1. Measuring Planck’s Constant Using Light Emitting Diodes (LED’s) Department of Physics and Astronomy Youngstown State University Dr. Michael Crescimanno Snowflake Kicovic

2. Purpose  To find an essentially simple, straightforward method for deriving Planck’s constant using a device that we can build.  This device has to be build easily. It should be durable and feasible.  The results yielded should give an accurate value for Planck’s constant.  This method, depending on the results, can then be used in an entry level physics lab, such as that of a high school physics lab.

3. Planck’s Constant  1900, Max Planck proposed discrete behavior for an object of subatomic dimensions - Planck’s constant h - the natural unit of action 6.626 x 10 -34 J-s, or kgm 2 /s  It also represents angular momentum.  1905, Einstein stated that electromagnetic radiation is localized in photons with frequency f and energy: E = hf  1913, Niels Bohr extended idea to electron existing between states of discrete energy. Transitions are accompanied by absorption or emission of photons with f = E/h.

4. The Photoelectric Effect  1902 it was proven that the KE max of an electron is independent of intensity of light ray and dependent on the frequency f.  1905, Einstein formed a fundamental theory where light is composed of photons = energy quanta.  Electrons are ejected (with great velocity from the atom) by the E of the photon.  Each light quantum consists of an amount of E = hf

5. Light Emitting Diodes  Light Emitting Diodes have p-n junctions where voltage yields a flow of current. The carriers (electrons and holes) are injected across the junction producing light.

6. Procedure  We first build the device, approximately taking 15 minutes. The device consists of 5 different colored LED’s, a 6 volt battery pack, a potentiometer, an on/off switch, a 330  resistor, a loose set of black and red wire, and a wire with an alligator clip.  The apparatus is turned on.  The alligator clip is attached to a LED lead.

7. Procedure (cont’d)  The loose wires (black and red) are connected to a Multimeter (which reads the voltage across the LED).  Turning the room lights off, we vary the voltage (with the potentiometer) to see the max voltage before shutoff of the LED.  We record the value.  After, we turn the potentiometer back to maximum, and we measure the wavelength of each diode with a spectrometer.

8. Apparatus

9. Circuit Diagram

10. The Setup

11. Blue Diode

12. Green Diode

13. Orange Diode

14. Large Red Diode

15. Small Red Diode

16. Data DiodeVoltage (V)Wavelenghts (  Blue2.196640 Green1.536695 Orange1.507695 Large Red1.530700 Small Red1.287680

17. Experimental Results  From before E = hf, therefore, we used the formula h = (e V  c  = 5.84 x 10 -34  h = (5.84 +  ) x 10 -34   n)  i n (h i – h) 2 ] -1  

18. Conclusion  Being that the  was 1.05, it is evident that the errors in the experiment were random rather than systematic.  This goes to show that this experiment is very effective and efficient, while at the same time being very simplistic.  These conclusions therefore exhibit the perfect characteristics for an entry level physics course while making it an interesting and EASY method for obtaining one of nature’s constants.

19. Questions?