Hands-on activities with LEDs and light

Nikolaos Voudoukis Sarantos Oikonomidis George Kalkanis

Science, Technology and Environment Laboratory,

Pedagogical Department P.E.,

University of Athens, Greece

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?

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.

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

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.

• 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.

• 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.

The basic equation Energy : E = h f = e V

Where• h = 6.63 * 10^-34 J sec Planck’s constant • f : frequency• e = 1.6 * 10^-19 Cb electron’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.

Implementation – design of the circuit

• Materials• battery 4,5 V• breadboard• cables• digital voltmeter• spectrometer• resistor 220Ω• five LEDs : red, orange,

yellow, green, blue.

The experiment – measurements

The color of the emitted light depends on the chemical composition of the semiconducting material used and can be near-ultraviolet, visible or infrared depends on the band gap energy of the materials forming the p-n junction (e.g. GaAs : gallium arsenide etc.).

Measurements of 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).

LED color Voltage across

LED (V)

Red 1.77

Orange 1.81

Yellow 1.91

Green 2.03

Blue 3.05

Finding the energy (an LED emit) from the voltage

Energy : E = eV

LED color Voltage across LED (V)

Energy (eV) Energy

(* 10^-19 J)

Red 1.77 1.77 2.83

Orange 1.81 1.81 2.90

Yellow 1.91 1.91 3.06

Green 2.03 2.03 3.25

Blue 3.05 3.05 4.88

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.

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.

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

Frequency: f = c / λ with (c=3 * 10^8 m/s)

LED color Wavelength λ (nm)

Frequency f (* 10^14 Hz)

Red 680 4.41

Orange 620 4.84

Yellow 580 5.17

Green 540 5.56

Blue 440 6.82

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

h = eV / f where e = 1.6 * 10^-19 CLED color Voltage across

LED (V)Frequency f (x

1014Hz)Plank’s constant

h (x 10-34 J s)

Red 1.77 4.41 6.42

Orange 1.81 4.84 5.98

Yellow 1.91 5.17 5.91

Green 2.03 5.56 5.84

Blue 3.05 6.82 7.16

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

e = h f / V where h = 6.63 * 10^-34 J secLED color Voltage across

LED (V)Frequency f (x

1014Hz)Electron’s

charge e (* 10-19 C)

Red 1.77 4.41 1.65

Orange 1.81 4.84 1.77

Yellow 1.91 5.17 1.79

Green 2.03 5.56 1.81

Blue 3.05 6.82 1.48

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.

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.

• 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.