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Measuring Planck’s Constant
Using Light Emitting Diodes
(LED’s)
Department of Physics and Astronomy
Youngstown State University
Dr. Michael Crescimanno
Snowflake Kicovic
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.
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 kgm2/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.
The Photoelectric Effect
 1902
it was proven that the KEmax 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
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.
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
330W 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.
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.
Apparatus
Circuit Diagram
The Setup
Blue Diode
Green Diode
Orange Diode
Large Red Diode
Small Red Diode
Data
Diode
Blue
Green
Orange
Large Red
Small Red
Voltage (V)
2.196
1.536
1.507
1.530
1.287
Wavelenghts (l)
640
695
695
700
680
Experimental Results

From before E = hf, therefore, we used the
formula
h = (e V l) /c
 <h> = 5.84 x 10-34
 h = (5.84 + s) x 10-34
 s = [ (1/n)Sin (hi – h)2]-1
 s = 1.05
Conclusion
that the s 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.
 Being
Questions?