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Measuring Planck’s Constant
Using Light Emitting Diodes
Department of Physics and Astronomy
Youngstown State University
Dr. Michael Crescimanno
Snowflake Kicovic
 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
 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.
 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.
Circuit Diagram
The Setup
Blue Diode
Green Diode
Orange Diode
Large Red Diode
Small Red Diode
Large Red
Small Red
Voltage (V)
Wavelenghts (l)
Experimental Results
From before E = hf, therefore, we used the
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
that the s was 1.05, it is evident that the
errors in the experiment were random rather than
 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