Download Hands-on Activities with LEDs and Light

Science, Technology and Environment Laboratory,
Pedagogical Department P.E.,
University of Athens, Greece
Hands-on activities with LEDs and light
Nikolaos Voudoukis
Sarantos Oikonomidis
George Kalkanis
The motive – The question
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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
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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=hf=eV
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
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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
LED color
Voltage
across
LED (V)
Red
1.77
Orange
1.81
Yellow
1.91
Green
2.03
Blue
3.05
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).
Finding the energy (an LED
emit) from the voltage
Energy :
LED color
E = eV
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 Frequency f
λ (nm)
(* 10^14 Hz)
Red
Orange
Yellow
Green
Blue
680
620
580
540
440
4.41
4.84
5.17
5.56
6.82
Calculation of Planck’s constant
(take for granted the value of electrons charge)
h = eV / f
LED color
where e = 1.6 * 10^-19 C
Voltage across
LED (V)
Frequency f (x Plank’s constant
1014Hz)
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=hf/V
LED color
where h = 6.63 * 10^-34 J sec
Voltage across
LED (V)
Frequency f (x
Electron’s
1014Hz)
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.