Physics Laboratory Activity 28a1
Determination of Planck's Constant
The purpose of this laboratory activity is to use three LEDs (i.e., Light Emitting Diodes) to
determine the value of Planck's constant. For additional information on LEDs see the AAPT/PTRA
Teacher Resource, Teaching about Lightwave Communications.
MATERIALS & PROCEDURES:
Voltmeter
LED
100-ohm
resistor
DC Power Supply
[ Diagram of Apparatus ]
Set up circuit shown in the diagram above. Apply small voltage to a 200-ohm resistor and LED
connected in series. Note positive side of LED has the longer lead. Record the voltage when the
LED just starts to shine. *NOTE* Do not exceed 3.0 volts with the blue LED, 2.5 volts with the
green LED, and 2.0 volts with red LED. See page 2.6 in AAPT/PTRA Teacher Resource Teaching
about Electric Circuits by Earl Feltyberger et. al.
DATA TABLE:
Voltage =
(Maximum - 2.0 volts)
Voltage =
(Maximum - 2.5 volts)
Voltage =
((Maximum - 3.0 volts)
Color of light is Red
Wavelength = 660 nm
Color of light is Green Wavelength = 570 nm
Color of light is Blue
Wavelength = 465 nm
If you do not have the equipment, there are simulations at
• http://www.walter-fendt.de/ph14e/photoeffect.htm and
• http://phet.colorado.edu/simulations/sims.php?sim=Photoelectric_Effect
1
From AAPT/PTRA Role of the Laboratory Teacher Resource Guide (Modified for use with
Arbor Scientific’s Electricity Energy Generator P6-8022)
Lab 28a - Planck's Constant
< Page 1 of 3>
© 2009 Jim & Jane Nelson
CALCULATIONS:
1.
Every coulomb of electrical charge, which flows through the LED, delivers a number of joules
of energy numerically equal to the voltage across the LED (i.e., volts = joules/coulomb). Given
that an electron has a charge equal to -1.6x10-19 coulombs, how much energy is delivered per
electron to produce a photon of red light? ... to produce a photon of green light? ... to produce
a photon of blue light? Show example of how you determined these values.
Red Light:
Green Light:
Blue Light:
2.
Give the equation that shows how the frequency of light and the speed of the light are related
to wavelength.
3.
What is the frequency of red light?
green light?
Show example of how you determined these values.
4.
Give the equation, which shows how energy per photon, Planck's constant, and frequency are
related.
5.
Solve for Planck's constant using your energy per photon and frequency values in 1 and 3
above. Give the average value of Planck's constant (i.e., h) with units. Show example of how
you determined these values.
blue light?
Using Red Light:
Using Green Light:
Using Blue Light:
6.
What is the energy of a photon with a wavelength of 450 nm?
7.
Plot a graph of energy to excite LEDs versus frequency of the LED. Write the equation for
this graph and explain the meaning of the slope, the y-axis intercept, and the x-axis intercept?
8.
How is this experiment similar to and different from the original photoelectric experiments
done by Hertz and Millikan?
Lab 28a - Planck's Constant
< Page 2 of 3>
© 2009 Jim & Jane Nelson
CONCLUSION:
What is the value of Planck’s constant that you determined?
Look up the accepted value of Planck’s constant and determine the percent error for your
measurements? Show how you determined your answer.
Is your average of your three values for "h" in agreement with the accepted value within one
power of ten (i.e., Within an order of magnitude)? Is you average too large or too small?
What could contribute to this error?
Lab 28a - Planck's Constant
< Page 3 of 3>
© 2009 Jim & Jane Nelson