Hands-on activities with LEDs and light

1. Science, Technology and Environment Laboratory, Pedagogical Department P.E., University of Athens, Greece Hands-on activities with LEDs and light NikolaosVoudoukis Sarantos Oikonomidis George Kalkanis

2. The motive – The question Is it possible for students to execute simple hands-on experiments with LEDs in order to find : • Planck’s constant, • electron’s charge, • the energy required to light the LED, • the frequency of light emitting from the LED?

3. Experimental procedure The experimental procedure consists of measurements with digital voltmeter and spectrometer. The voltmeter is used to measure the voltage across the leads of the LED. The spectrometer is used to estimate the wavelength corresponding to the maximum intensity of the light emitted from the LED.

4. Theoretical framework .The activity is proposed for the students of High school that have been taught the nature of light and basic elements of Quantum Physics (photons, Planck’s constant etc). Nevertheless it is necessary a theoretical framework as an introductory fundamental lesson-material for LEDs and their way of light emission

5. What is LED? Light Emitting Diode (LED) is a special diode that emits light when connected in a circuit and biased in the forward direction. In other words it is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction.

6. The phenomenon which takes place is the spontaneous emission of radiation in the visible and infrared regions of the spectrum from a forward biased p-n junction.

7. The normally empty conduction band of the semiconductor is populated by electrons injected into it by the forward current through the junction and light is generated when these electrons recombine with holes in the valence band to emit a photon.

8. The basic equationEnergy : E = h f = e V Where • h = 6.63 * 10^-34 J sec Planck’s constant • f : frequency • e = 1.6 * 10^-19 Cbelectron’s charge • V : voltage across LED • The energy E is released with the creation of a photon. Also the energy E of light emitted by a LED is related with e and V is approximately equal to the band gap energy of the semiconductor.

9. Implementation – design of the circuit • Materials • battery 4,5 V • breadboard • cables • digital voltmeter • spectrometer • resistor 220Ω • five LEDs : red, orange, yellow, green, blue.

10. The experiment – measurements

11. Measurementsof voltage across LED We measured with the voltmeter the voltage across the leads of the LED (each of these five LEDs). So we constructed a data table (shown above).

12. Finding the energy (an LED emit) from the voltage

13. The spectrometer The spectrometer can be used to examine the light from the LED and to estimate the peak wavelength of the light emitted by the LED.

14. The spectrometer has a plastic disk with an attached diffraction grating that can be rotated. Looking at the spectrometer we can see a continuous spectrum of colors from red through violet. Looking at an LED we read the number on the scale corresponding to the light.

15. Estimation of wavelength with use of spectrometer and calculation of the corresponding frequency

16. Calculation of Planck’s constant(take for granted the value of electrons charge)

17. Calculation of electron’s charge(take for granted the value of Planck’s constant)

18. Verification • We take e = 1.6 * 10-19 C and • h = 6.63 * 10-34 J s • With use of V measurements we calculate the frequencies • The results are very close to the experimental values.

19. Conclusion • The experiments are successful because the experimental values and the correlated results are very close to the theoretical values. • Also these experiments are very simple hands-on experiments that can be executed by students.

20. The above procedure was applied to the students of Pedagogical Department of University of Athens • The results were satisfying and the activities will be included in laboratory exercises for the students in the academic year of 2007.